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THE    INVENTIONS 
RESEARCHES    AND    WRITINGS 

OF 

NIKOLA    TESLA 


TO  HIS  COUNTRYMEN 


IN      EASTERN      EUROPE     THIS     RECORD     OF 
THE    WORK    ALREADY     ACCOMPLISHED     BY 

NIKOLA  TESLA 

IS    RESPECTFULLY     DEDICATED 


THE     INVENTIONS 
RESEARCHES    AND    WRITINGS 

OF 

NIKOLA  TESLA 


WITH   SPECIAL   REFERENCE   TO  HIS  WORK   IN    POLYPHASE 
CURRENTS  AND  HIGH   POTENTIAL  LIGHTING 


HY 


THOMAS  COMMERFORD   MARTIN 

Editor   THE   ELKCTHICAL    ENGiNEtR ;    Past-President    American    Institute   Electrical   Engineers 


1894 

THE   ELECTRICAL   ENGINEER 

NEW    YORK 

D.  VAN  NOSTRAND  COMPANY, 
NEW  YORK. 


Entered  according  to  Act  of  Congress  in  the  year  1893  by 

T.  C.  MARTIN 
in  the  office  of  the  Librarian  of  Congress  at  Washington 


Press  of  Mcllroy  &  Emmet,  36  Cortlandt  St.,  N.  Y. 


PREFACE. 


rpHE  electrical  problems  of  the  present  day  lie  largely  in  the 
**  economical  transmission  of  power  and  in  the  radical  im- 
provement of  the  means  and  methods  of  illumination.  To  many 
workers  and  thinkers  in  the  domain  of  electrical  invention,  the 
apparatus  and  devices  that  are  familiar,  appear  cumbrous  and 
wasteful,  and  subject  to  severe  limitations.  They  believe  that 
the  principles  of  current  generation  must  be  changed,  the  area 
of  current  supply  be  enlarged,  and  the  appliances  used  by  the 
consumer  be  at  once  cheapened  and  simplified.  The  brilliant 
successes  of  the  past  justify  them  in  every  expectancy  of  still 
more  generous  fruition. 

The  present  volume  is  a  simple  record  of  the  pioneer  work 
done  in  such  departments  up  to  date,  by  Mr.  Nikola  Tesla,  in 
whom  the  world  has  already  recognized  one  of  the  foremost  of 
modern  electrical  investigators  and  inventors.  No  attempt  what- 
ever has  been  made  here  to  emphasize  the  importance  of  his 
researches  and  discoveries.  Great  ideas  and  real  inventions  win 
their  own  way,  determining  their  own  place  by  intrinsic  merit. 
But  with  the  conviction  that  Mr.  Tesla  is  blazing  a  patli  that 
electrical  development  must  follow  for  many  years  to  come,  the 
compiler  has  endeavored  to  bring  together  all  that  bears  the  im- 
press of  Mr.  Tesla's  genius,  and  is  worthy  of  preservation.  Aside 
from  its  value  as  showing  the  scope  of  his  inventions,  this 
volume  may  be  of  service  as  indicating  the  range  of  his  thought. 
There  is  intellectual  profit  in  studying  the  push  and  play  of  a 
vigorous  and  original  mind. 

Althqugh  the  lively  interest  of  the  public  in  Mr.  Tesla's  work 
is  perhaps  of  recent  growth,  this  volume  covers  the  results  of 
full  ten  years.  It  includes  his  lectures,  miscellaneous  articles 


vi  PREFACE. 

and  discussions,  and  makes  note  of  all  his  inventions  thus  far 
known,  particularly  those  bearing  on  polyphase  motors  and  the 
effects  obtained  with  currents  of  high  potential  and  high  fre- 
quency. It  will  be  seen  that  Mr.  Tesla  has  ever  pressed  forward, 
barely  pausing  for  an  instant  to  work  out  in  detail  the  utilizations 
that  have  at  once  been  obvious  to  him  of  the  new  principles  he 
has  elucidated.  Wherever  possible  his  own  language  has  been 
employed. 

It  may  be  added  that  this  volume  is  issued  with  Mr.  Tesla's 
sanction  and  approval,  and  that  permission  has  been  obtained  for 
the  re-publication  in  it  of  such  papers  as  have  been  read  before 
various  technical  societies  of  this  country  and  Europe.  Mr. 
Tesla  has  kindly  favored  the  author  by  looking  over  the  proof 
sheets  of  the  sections  embodying  his  latest  researches.  The 
Work  has  also  enjoyed  the  careful  revision  of  the  author's 
friend  and  editorial  associate,  Mr.  Joseph  Wetzler,  through 
whose  hands  all  the  proofs  have  passed. 

DECEMBER,  1893.  T.  C.  M. 


CONTENTS. 


PART  I, 

POLYPHASE  CUERENTS. 

CHAPTEK  I. 

BIOGRAPHICAL  AND    INTRODUCTORY.. 


CHAPTER  II. 

A  NEW   SYSTEM   OF   ALTERNATING   CURRENT  MOTORS  AND 

TRANSFORMERS Y 

CHAPTER  III. 

THE  TESLA  ROTATING  MAGNETIC  FIELD. — MOTORS  WITH 
CLOSED  CONDUCTORS. — SYNCHRONIZING  MOTORS. — ROTA- 
TING FIELD  TRANSFORMERS 9 

CHAPTER  IV. 

MODIFICATIONS  AND  EXPANSIONS  OF  THE  TESLA  POLYPHASE 

SYSTEMS 26 

CHAPTER  V. 

UTILIZING  FAMILIAR  TYPES  OF  GENERATORS  OF  THE  CON- 
TINUOUS CURRENT  TYPE 31 

CHAPTER  VI. 

METHOD     OF     OBTAINING    DESIRED    SPEED    OF    MOTOR    OR 

GENERATOR 36 


viii 

CHAPTER  VII. 

REGULATOR  FOR  ROTARY  CURRENT  MOTORS 45 

CHAPTER  VIII. 
SINGLE  CIRCUIT,  SELF-STARTING  SYNCHRONIZING  MOTORS.  . .     50 

CHAPTER  IX. 

CHANGE    FROM    DOUBLE    CURRENT    TO     SINGLE     CURRENT 

MO.TORS 56 

CHAPTER  X. 

MOTOR  WITH  "  CURRENT  LAG  "  ARTIFICIALLY  SECURED 58 

CHAPTER  XL 

ANOTHER  METHOD  OF  TRANSFORMATION  FROM  A  TORQUE  TO 

A  SYNCHRONIZING  MOTOR ...    62 

CHAPTER  XII. 
"  MAGNETIC    LAG  "  MOTOR 67 

CHAPTER  XIII. 

METHOD  OF   OBTAINING    DIFFERENCE   OF   PHASE    BY   MAG- 
NETIC SHIELDING 71 

CHAPTER  XIV. 
TYPE  OF  TESLA  SINGLE-PHASE  MOTOR 76 

CHAPTER  XV. 

MOTORS  WITH    CIRCUITS  OF  DIFFERENT  RESISTANCE 79 

CHAPTER  XVI. 

MOTOR   WITH    EQUAL   MAGNETIC   ENERGIES   IN    FIELD  AND 

ARMATURE g^ 

CHAPTER  XVII. 

MOTORS  WITH    COINCIDING   MAXIMA   OK    MAGNETIC    EFFECT 

IN  ARMATURE   AND  FIELD 83 


CONTENTS.  ix 

CHAPTER  XVIII. 

MOTOR  BASED  ON  THE  DIFFERENCE  OF  PHASE  IN  THE  MAG- 
NETIZATION OF  THE  INNER  AND  OUTER  PARTS  OF  AN 
IRON  CORE 88 

CHAPTER  XIX. 
ANOTHER  TYPE  OF  TESLA  INDUCTION  MOTOR 92 

CHAPTER  XX. 

COMBINATIONS    OF    SYNCHRONIZING     MOTOR     AND     TORQUE 

MOTOR 95 

CHAPTER  XXI. 

MOTOR  WITH  A  CONDENSER  IN  THE  ARMATURE  CIRCUIT  . . .   101 

CHAPTER  XXII. 
MOTOR  WITH  CONDENSER  IN  ONE  OF  THE  FIELD  CIRCUITS.   106 

CHAPTER  XXIII. 

TESLA  POLYPHASE  TRANSFORMER 1 J9 

CHAPTER  XXIV. 

A    CONSTANT    CURRENT     TRANSFORMER     WITH     MAGNETIC 

SHIELD  BETWEEN   COILS  OF  PRIMARY  AND  SECONDARY  113 


PART  II. 

THE     TESLA    EFFECTS     WITH     HIGH    FREQUENCY    AND 
HIGH  POTENTIAL  CURRENTS. 

CHAPTER  XXV. 

INTRODUCTORY. — THE    SCOPE  OF  THE  TESLA  LECTURES 119 

CHAPTER  XXVI. 

THE  NEW  YORK  LECTURE.  EXPERIMENTS  WITH  ALTERNATE 
CURRENTS  OF  VERY  HIGH  FREQUENCY,  AND  THEIR 
APPLICATION  TO  METHODS  OF  ARTIFICIAL  ILLUMINA- 
TION, MAY  20,  1S91 145 


x  CONTENTS. 

CHAPTER  XXVII. 

THE    LONDON   LECTURE.      EXPERIMENTS    WITH    ALTERNATE  • 
CURRENTS  OF  HIGH  POTENTIAL  AND  HIGH  FREQUENCY, 
FEBRUARY  3,  1892 198 

CHAPTER  XXVIII. 

THE  PHILADELPHIA  AND  ST.  Louis  LECTURE.  ON  LIGHT 
AND  OTHER  HIGH  FREQUENCY  PHENOMENA,  FEBRUARY 
AND  MARCH,  1893. 294 

CHAPTER  XXIX. 

TESLA    ALTERNATING    CURRENT    GENERATORS     FOR    HIGH 

FREQUENCY 374 

CHAPTER  XXX. 
ALTERNATE  CURRENT  ELECTROSTATIC  INDUCTION  APPARATUS  392 

CHAPTER  XXXI. 

"  MASSAGE  "  WITH  CURRENTS  OF  HIGH  FREQUENCY 394 

CHAPTER  XXXII. 

ELECTRIC  DISCHARGE  IN  VACUUM  TUBES 396 


PART  III. 

MISCELLANEOUS  INVENTIONS  AND  WEITINGS. 

CHAPTER  XXXIII. 

METHOD   OF   OBTAINING   DIRECT   FROM  ALTERNATING  CUR- 
RENTS     409 

CHAPTER  XXXIV. 
CONDENSERS  WITH  PLATES  IN  OIL 418 

CHAPTER  XXXV. 

ELECTROLYTIC  REGISTERING   ]!UETER .   420 


CONTENTS.  xi 

CHAPTER  XXXVI. 

THERMO-MAGNETIC    MOTORS    AND    PYRO-MAGNETIC    GENE- 
RATORS  , 424 

CHAPTER  XXXVII. 
ANTI-SPARKING  DYNAMO  BRUSH  AND  COMMUTATOR 432 

CHAPTER  XXXVIII. 

AUXILIARY   BRUSH   REGULATION   OF  DIRECT  CURRENT   DY- 
NAMOS  438 

CHAPTER  XXXIX. 

IMPROVEMENT  IN  DYNAMO  AND  MOTOR  CONSTRUCTION 448 

CHAPTER  XL. 
TESLA   DIRECT  CURRENT   ARC  LIGHTING  SYSTEM 451 

CHAPTER  XLI. 

IMPROVEMENT  IN  UNIPOLAR  GENERATORS  .  .  .   465 


PART  IV. 

APPENDIX  :     EARLY  PHASE  MOTORS  AND  THE    TESLA 
OSCILLATORS. 

CHAPTER  XLIL 

MR.  TESLA'S  PERSONAL  EXHIBIT  AT  THE  WORLD'S  FAIR  . . .   4.77 

CHAPTER  XLIII. 

THE  TESLA  MECHANICAL  AND  ELECTRICAL  OSCILLATORS...   486 


PART  I. 


POLYPHASE  CURRENTS. 


CHAPTER  I. 

BIOGRAPHICAL  AND  INTRODUCTORY. 

As  AN  introduction  to  the  record  contained  in  this  volume 
of  Mr.  Tesla' s  investigations  and  discoveries,  a  few  words  of  "a 
biographical  nature  will,  it  is  deemed,  not  be  out  of  place,  nor 
other  than  welcome. 

Nikola  Tesla  was  born  in  1857  at  Smiljan,  Lika,  a  borderland 
region  of  Austro-Hungary,  of  the  Serbian  race,  wrhich  has  main- 
tained against  Turkey  and  all  comers  so  unceasing  a  struggle  for 
freedom.  His  family  is  an  old  and  representative  one  among 
these  Switzers  of  Eastern  Europe,  and  his  father  was  an  eloquent 
clergyman  in  the  Greek  Church.  An  uncle  is  to-day  Metropoli- 
tan in  Bosnia.  His  mother  was  a  woman  of  inherited  ingenuity, 
and  delighted  not  only  in  skilful  work  of  the  ordinary  household 
character,  but  in  the  construction  of  such  mechanical  appliances 
as  looms  and  churns  and  other  machinery  required  in  a  rural 
community.  Nikola  was  educated  at  Gospich  in  the  public 
school  for  four  years,  and  then  spent  three  years  in  the  Real 
Scliule.  He  was  then  sent  to  Carstatt,  Croatia,  where  he  con- 
tinued his  studies  for  three  years  in  the  Higher  Real  Scliule. 
There  for  the  first  time  he  saw  a  steam  locomotive.  He  gradu- 
ated in  1873,  and,  surviving  an  attack  of  cholera,  devoted  him- 
self to  experimentation,  especially  in  electricity  and  magnetism. 
His  father  would  have  had  him  maintain  the  family  tradition  by 
Altering  the  Church,  but  native  genius  was  too  strong,  and  he 
was  allowed  to  enter  the  Polytechnic  School  at  Gratz,  to  finish 
his  studies,  and  with  the  object  of  becoming  a  professor  of  math- 
ematics and  physics.  One  of  the  machines  there  experimented 
with  was  a  Gramme  dynamo,  used  as  a  motor.  Despite  his  in- 
structor's perfect  demonstration  of  the  fact  that  it  \vas  impossible 
to  operate  a  dynamo  without  commutator  or  brushes,  Mr.  Tesla 
could  not  be  convinced  that  such  accessories  \\eiv  K-cessary  or 
desirable.  He  had  already  seen  with  quick  intuition  that  a  way 
could  be  found  to  dispense  with  them  ;  and  from  that  time  he  may 


4  INVENTIONS  OF  NIKOLA  TESLA. 

be  said  to  have  begun  work  on  the  ideas  that  fructified  ultimately 
in  his  rotating  field  motors. 

In  the  second  year  of  his  Gratz  course,  Mr.  Tesla  gave  up  the 
notion  of  becoming  a  teacher,  and  took  up  the  engineering  cur- 
riculum. His  studies  ended,  he  returned  home  in  time  to  see  his 
father  die,  and  then  went  to  Prague  and  Buda-Pesth  to  study 
languages,  with  the  object  of  qualifying  himself  broadly  for  the 
practice  of  the  engineering  profession.  For  a  short  time  he 
served  as  an  assistant  in  the  Government  Telegraph  Engineer- 
ing Department,  and  then  became  associated  with  M.  Puskas,  a 
personal  and  family  friend,  and  other  exploiters  of  the  telephone 
in  Hungary.  He  made  a  number  of  telephonic  inyentions,  but 
found  his  opportunities  of  benefiting  by  them  limited  in  various 
ways.  To  gain  a  wider  field*  of  action,  he  pushed  on  to  Paris 
and  there  secured  employment  as  an  electrical  engineer  with  one 
of  the  large  companies  in  the  new  industry  of  electric  lighting. 

It  was  during  this  period,  and  as  early  as  1882,  that  he  began 
serious  and  continued  efforts  to  embody  the  rotating  field  prin- 
ciple' in  operative  apparatus.  He  was  enthusiastic  about  it ;  be- 
lieved it  to  mark  a  new  departure  in  the  electrical  arts,  and  could 
think  of  nothing  else.  In  fact,  but  for  the  solicitations  of  a  few 
friends  in  commercial  circles  who  urged  him  to  form  a  company 
to  exploit  the  invention,  Mr.  Tesla,  then  a  youth  of  little  worldly 
experience,  would  have  sought  an  immediate  opportunity  to  pub- 
lish his  ideas,  believing  them  to  be  worthy  of  note  as  a  novel  and 
radical  advance  in  electrical  theory  as  well  as  destined  to  have 
a  profound  influence  on  all  dynamo  electric  machinery. 

At  last  he  determined  that  it  would  be  best  to  try  his  fortunes 
in  America.  In  France  he  had  met  many  Americans,  and  in 
contact  with  them  learned  the  desirability  of  turning  every  new 
idea  in  electricity  to  practical  use.  He  learned  also  of  the  ready 
encouragement  given  in  the  United  States  to  any  inventor  who 
could  attain  some  new  and  valuable  result.  The  resolution  \v.  .., 
formed  with  characteristic  quickness,  and  abandoning  all  his 
prospects  in  Europe,  he  at  once  set  his  face  westward. 

Arrived  in  the  United  States,  Mr.  Tesla  took  off  his  coat  tin- 
day  he  arrived,  in  the  Edison  Works.  That  place  had  been  a- 
goal  of  his  ambition,  and  one  can  readily  imagine  the  benefit  and 
stimulus  derived  from  association  with  Mr.  Edison,  for  whom 
Mr.  Tesla  'aas  always  had  the  strongest  admiration.  It  was  im- 
possible, however,  that,  with  -his  own  ideas  to  carry  out,  and  his 


POLYPHASE  CURRENTS.  5 

own  inventions  to  develop,  Mr.  Tesla  could  long  remain  in  even 
the  most  delightful  employ ;  and,  his  work  now  attracting  atten- 
tion, he  left  the  Edison  ranks  to  join  a  company  intended  to 
make  and  sell  an  arc  lighting  system  based  on  some  of  his  inven- 
tions in  that  branch  of  the  art.  With  unceasing  diligence  he 
brought  the  system  to  perfection,  and  saw  it  placed  on  the  market. 
But  the  thing  which  most  occupied  his  time  and  thoughts,  how- 
ever, all  through  this  period,  was  his  old  discovery  of  the  rotating 
field  principle  for  alternating  current  work,  and  the  application 
of  it  in  motors  that  have  now  become  known  the  world  over. 

Strong  as  his  convictions  on  the  subject  then  were,  it  is  a  fact , 
that  he  stood  very  much  alone,  for  the  alternating  current  had 
no  well  recognized  place.  Few  electrical  engineers  had  ever 
used  it,  and  the  majority  were  entirely  unfamiliar  with  its  value, 
or  even  its  essential  features.  Even  Mr.  Tesla  himself  did  not, 
until  after  protracted  effort  and  experimentation,  learn  how  to 
construct  alternating  current  apparatus  of  fair  efficiency.  But 
that  he  had  accomplished  his  purpose  was  shown  by  the  tests  of 
Prof.  Anthony,  made  in  the  of  winter  188T-8,  when  Tesla  motors 
in  the  hands  of  that  distinguished  expert  gave  an  efficiency  equal 
to  that  of  direct  current  motors.  Nothing  now  stood  in  the  way 
of  the  commercial  development  and  introduction  of  such  motors, 
except  that  they  had  to  be  constructed  with  a  view  to  operating 
on  the  circuits  then  existing,  which  in  this  country  were  all  of 
high  frequency. 

The  first  full  publication  of  his  work  in  this  direction — outside 
his  patents — was  a  paper  read  before  the  American  Institute  of 
Electrical  Engineers  in  New  York,  in  May,  1888  (read  at  the 
suggestion  of  Prof.  Anthony  and  the  present  writer),  when  he 
exhibited  motors  that  had  been  in  operation  long  previous,  and 
with  which  his  belief  that  brushes  and  commutators  could  be 
dispensed  with,  was  triumphantly  proved  to  be  correct.  The 
section  of  this  volume  devoted  to  Mr.  Tesla's  inventions  in  the 
utilization  of  polyphase  currents  will  show  how  thoroughly  from 
the  outset  he  had  mastered  the  fundamental  idea  and  applied  it 
in  the  greatest  variety  of  ways. 

Having  noted  for  years  the  many  advantages  obtainable  with 
alternating  currents,  Mr.  Tesla  was  naturally  led  on  to  experi- 
ment with  them  at  higher  potentials  and  higher  frequencies  than 
were  common  or  approved  of.  Ever  pressing  forward  to  deter- 
mine in  even  the  slightest  degree  the  outlines  of  the  unknown,  he 


6  INVENTIONS  OF  NIKOLA  TESLA. 

was  rewarded  very  quickly  in  this  field  with  results  of  the  most- 
surprising  nature.  A  slight  acquaintance  with  some  of  these 
experiments  led  the  compiler  of  this  volume  to  urge  Mr.  Tesla 
to  repeat  them  before  the  American  Institute  of  Electrical  En- 
gineers. This  was  done  in  May,  1891,  in  a  lecture  that  marked, 
beyond  question,  a  distinct  departure  in  electrical  theory  and 
practice,  and  all  the  results  of  which  have  not  yet  made  them- 
selves fully  apparent.  The  New  York  lecture,  and  its  suc- 
cessors, two  in  number,  are  also  included  in  this  volume,  with  a 
few  supplementary  notes. 

Mr.  Tesla's  work  ranges  far  beyond  the  vast  departments  of 
polyphase  currents  and  high  potential  lighting.  The  "  Miscella- 
neous "  section  of  this  volume  includes  a  great  many  other  in- 
ventions in  arc  lighting,  transformers,  pyro-magnetic  generators, 
thermo-magnetic  motors,  third-brush  regulation,  improvements 
in  dynamos,  new  forms  of  incandescent  lamps,  electrical  meters, 
condensers,  unipolar  dynamos,  the  conversion  of  alternating  into 
direct  currents,  etc.  It  is  needless  to  say  that  at  this  moment 
Mr.  Tesla  is  engaged  on  a  number  of  interesting  ideas  and  inven- 
tions, to  be  made  public  in  due  course.  The  present  volume 
deals  simply  with  his  work  accomplished  to  date. 


CHAPTER  II. 

A  NEW  SYSTEM  OF  ALTERNATING  CURRENT  MOTORS  AND 
TRANSFORMERS. 

THE  present  section  of  this  volume  deals  with  polyphase  cur- 
rents, and  the  inventions  by  Mr.  Tesla,  made  known  thus  far,  in 
which  he  has  embodied  one  feature  or  another  of  the  broad 
principle  of  rotating  field  poles  or  resultant  attraction  exerted  on 
the  armature.  It  is  needless  to  remind  electricians  of  the  great 
interest  aroused  by  the  first  enunciation  of  the  rotating  field 
principle,  or  to  dwell  upon  the  importance  of  the  advance  from 
a  single  alternating  current,  to  methods  and  apparatus  which  deal 
with  more  than  one.  Simply  prefacing  the  consideration  here 
attempted  of  the  subject,  with  the  remark  that  in  nowise  is  the 
object  of  this  volume  of  a  polemic  or  controversial  nature,  it 
may  be  pointed  out  that  Mr.  Tesla's  work  has  not  at  all  been 
fully  understood  or  realized  up  to  date.  To  many  readers,  it  is 
believed,  the  analysis  of  what  he  has  done  in  this  department 
will  be  a  revelation,  while  it  will  at  the  same  time  illustrate  the 
beautiful  flexibility  and  range  of  the  principles  involved.  It 
will  be  seen  that,  as  just  suggested,  Mr.  Tesla  did  not  stop  short 
at  a  mere  rotating  field,  but  dealt  broadly  with  the  shifting  of 
the  resultant  attraction  of  the  magnets.  It  will  be  seen  that  he 
went  on  to  evolve  the  "  multiphase  "  system  with  many  ramifica- 
tions and  turns;  that  he  showed  the  broad  idea  of  motors  em- 
ploying currents  of  differing  phase  in  the  armature  with  direct 
currents  in  the  field ;  that  he  first  described  and  worked  out  the 
idea  of  an  armature  with  a  body  of  iron  and  coils  closed  upon 
themselves ;  that  he  worked  out  both  synchronizing  and  torque 
motors;  that  he  explained  and  illustrated  how  machines  of  ordi- 
nary construction  might  be  adapted  to  his  system ;  that  he  em- 
ployed condensers  in  field  and  armature  circuits,  and  went  to  the 
bottom  of  the  fundamental  principles,  testing,  approving  or  reject- 
ing, it  would  appear,  every  detail  that  inventive  ingenuity  could 
hit  upon. 


8  INVENTIONS  OF  NIKOLA  TESLA. 

Now  that  opinion  is  turning  so  emphatically  in  favor  of  lower 
frequencies,  it  deserves  special  note  that  Mr.  Tesla  early  re- 
cognized the  importance  of  the  low  frequency  feature  in  motor 
work.  In  fact  his  first  motors  exhibited  publicly — and  which,  as 
Prof.  Anthony  showed  in  his  tests  in  the  winter  of  1887-8,  were 
the  equal  of  direct  current  motors  in  efficiency,  output  and  start- 
ing torque — were  of  the  low  frequency  type.  The  necessity 
arising,  however,  to  utilize  these  motors  in  connection  with  the 
existing  high  frequency  circuits,  our  survey  reveals  in  an  inter- 
esting manner  Mr.  Tesla's  fertility  of  resource  in  this  direction. 
But  that,  after  exhausting  all  the  possibilities  of  this  field,  Mr. 
Tesla  returns  to  low  frequencies,  and  insists  on  the  superiority  of 
his  polyphase  system  in  alternating  current  distribution,  need  not 
at  all  surprise  us,  in  view  of  the  strength  of  his  convictions,  so 
often  expressed,  on  this  subject.  This  is,  indeed,  significant,  and 
may  be  regarded  as  indicative  of  the  probable  development  next 
to  be  witnessed. 

Incidental  reference  has  been  made  to  the  efficiency  of  rotating 
field  motors,  a  matter  of  much  importance,  though  it  is  not  the 
intention  to  dwell  upon  it  here.  Prof.  Anthony  in  his  remarks 
before  the  American  Institute  of  Electrical  Engineers,  in  May, 
1888,  on  the  two  small  Tesla  motors  then  shown,  which  he  had 
tested,  stated  that  one  gave  an  efficiency  of  about  50  per  cent, 
and  the  other  a  little  over  sixty  per  cent.  In  1889,  some  tests 
were  reported  from  Pittsburgh,  made  by  Mr.  Tesla  and  Mr. 
Albert  Schmid,  on  motors  up  to  10  H.  p.  and  weighing  about 
850  pounds.  These  machines  showed  an  efficiency  of  nearly  90 
per  cent.  With  some  larger  motors  it  was  then  found  practic- 
able to  obtain  an  efficiency,  with  the  three  wire  system,  up  to  as 
high  as  94  and  95  per  cent.  These  interesting  figures,  which,  of 
course,  might  be  supplemented  by  others  more  elaborate  and  of 
later  date,  are  cited  to  show  that  the  efficiency  of  the  system  has 
not  had  to  wait  until  the  present  late  day  for  any  demonstration 
of  its  commercial  usefulness.  An  invention  is  none  the  less  beauti- 
ful because  it  may  lack  utility,  but  it  must  be  a  pleasure  to  any 
inventor  to  know  that  the  ideas  he  is  advancing  are  fraught  with 
substantial  benefits  to  the  public. 


CHAPTEE  III. 

THE  TESLA  KOTATING  MAGNETIC  FIELD. — MOTORS  WITH  CLOSED 
CONDUCTORS. — SYNCHRONIZING  MOTORS. — KOTATING  FIELD 
TRANSFORMERS. 

THE  best  description  that  can  be  given  of  what  he  attempted, 
and  succeeded  in  doing,  with  the  rotating  magnetic  field,  is  to  be 
found  in  Mr.  Tesla's  brief  paper  explanatory  of  his  rotary  cur- 
rent, polyphase  system,  read  before  the  American  Institute  of 
Electrical  Engineers,  in  New  York,  in  May,  1888,  under  the 
title  "  A  New  System  of  Alternate  Current  Motors  and  Trans- 
formers." As  a  matter  of  fact,  which  a  perusal  of  the  paper 
will  establish,  Mr.  Tesla  made  no  attempt  in  that  paper  to  de- 
scribe all  his  work.  It  dealt  in  reality  with  the  few  topics  enu- 
merated in  the  caption  of  this  chapter.  Mr.  Tesla's  reticence 
was  no  doubt  due  largely  to  the  fact  that  his  action  was  gov- 
erned by  the  wishes  of  others  with  whom  lie  was  associated,  but 
it  may  be  worth  mention  that  the  compiler  of  this  volume — who 
had  seen  the  motors  running,  and  who  was  then  chairman  of  the 
Institute  Committee  on  Papers  and  Meetings — had  great  diffi- 
culty in  inducing  Mr.  Tesla  to  give  the  Institute  any  paper  at  all. 
Mr.  Tesla  was  overworked  and  ill,  and  manifested  the  greatest 
reluctance  to  an  exhibition  of  his  motors,  but  his  objections  were 
at  last  overcome.  The  paper  was  written  the  night  previous  to 
the  meeting,  in  pencil,  very  hastily,  and  under  the  pressure 
just  mentioned. 

In  this  paper  casual  reference  was  made  to  two  special  forms 
of  motors  not  within  the  group  to  be  considered.  These  two 
forms  were  :  1.  A  motor  with  one  of  its  circuits  in  series  with  a 
transformer,  and  the  other  in  the  secondary  of  the  transformer. 
2.  A  motor  having  its  armature  circuit  connected  to  the  gener- 
ator, and  the  field  coils  closed  upon  themselves.  The  paper  in 
its  essence  is  as  follows,  dealing  witli  a  few  leading  features  of 
the  Tesla  system,  namely,  the  rotating  magnetic  field,  motors 


10  INVENTIONS  OF  NIKOLA  TESLA. 

with  closed  conductors,  synchronizing  motors,  and  rotating  field 
transformers : — 

The  subject  which  I  now  have  the  pleasure  of  bringing  to 
your  notice  is  a  novel  system  of  electric  distribution  and  trans- 
mission of  power  by  means  of  alternate  currents,  affording  pecu- 
liar advantages,  particularly  in  the  way  of  motors,  which  I  am 
confident  will  at  once  establish  the  superior  adaptability  of  these 
currents  to  the  transmission  of  power  and  will  show  that  many 
results  heretofore  unattainable  can  be  reached  by  their  use  ;  re- 
sults which  are  very  much  desired  in  the  practical  operation  of 
such  systems,  and  which  cannot  be  accomplished  by  means  of 
continuous  currents. 

Before  going  into  a  detailed  description  of  this  system,  I  think 
it  necessary  to  make  a  few  remarks  with  reference  to  certain  con- 
ditions existing  in  continuous  current  generators  and  motors, 
which,  although  generally  known,  are  frequently  disregarded. 

In  our  dynamo  machines,  it  is  well  known,  we  generate  alter- 
nate currents  which  we  direct  by  means  of  a  commutator,  a  com- 
plicated device  and,  it  may  be  justly  said,  the  source  of  most  of 
the  troubles  experienced  in  the  operation  of  the  machines.  Now, 
the  currents  so  directed  cannot  be  utilized  in  the  motor,  but 
they  must — again  by  means  of  a  similar  unreliable  device — 
be  reconverted  into  their  original  state  of  alternate  currents.^ 
The  function  of  the  commutator  is  entirely  external,  and  in  no 
way  does  it  affect  the  internal  working  of  the  machines.  In 
reality,  therefore,  all  machines  are  alternate  current  machines, 
the  currents  appearing  as  continuous  only  in  the  external  circuit 
during  their  transit  from  generator  to  motor.  In  view  simply  of 
this  fact,  alternate  currents  would  commend  themselves  as  a  more 
direct  application  of  electrical  energy,  and  the  employment  of 
continuous  currents  would  only  be  justified  if  we  had  dynamos 
which  would  primarily  generate,  and  motors  which  would  be 
directly  actuated  by,  such  currents. 

But  the  operation  of  the  commutator  on  a  motor  is  twofold  ; 
first,  it  reverses  the  currents  through  the  motor,  and  secondly, 
it  effects  automatically,  a  progressive  shifting  of  the  poles  of  one 
of  its  magnetic  constituents.  Assuming,  therefore,  that  both  of 
the  useless  operations  in  the  systems,  that  is  to  say,  the  directing 
of  the  alternate  currents  on  the  generator  and  reversing  the  direct 
currents  on  the  motor,  be  eliminated,  it  would  still  be  necessary, 
in  order  to  cause  a  rotation  of  the  motor,  to  produce  a  progressive 


POLYPHASE  CURRENTS. 


11 


shifting  of  the  poles  of  one  of  its  elements,  and  the  question 
presented  itself — How  to  perform  this  operation  by  the  direct 
action  of  alternate  currents  ?  I  will  now  proceed  to  show  how 
this  result  was  accomplished. 

In  the  first  experiment  a  drum-armature  was  provided  with 


Fie.  l. 


FIG.  la. 


two  coils  at  right  angles  to  each  other,  and  the  ends  of  these  coils 
were  connected  to  two  pairs  of  insulated  contact-rings  as  usual. 
A  ring  was  then  made  of  thin  insulated  plates  of  sheet-iron  and 
wound  with  four  coils,  each  two  opposite  coils  being  connected 
together  so  as  to  produce  free  poles  on  diametrically  opposite 
sides  of  the  ring.  The  remaining  free  ends  of  the  coils  were  then 
connected  to  the  contact-rings  of  the  generator  armature  so  as 
to  form  two  independent  circuits,  as  indicated  in  Fig.  9.'  It 
may  now  be  seen  what  results  were  secured  in  this  combination, 
and  witli  this  view  I  would  refer  to  the  diagrams,  Figs.  1  to  8#. 
The  field  of  the  generator  being  independently  excited,  the  rota- 
tion of  the  armature  sets  up  currents  in  the  coils  c  cl5  varying  in 


FIG 


FIG.  2a. 


strength  and  direction  in  the  well-known  manner.  In  the  posi- 
tion shown  in  Fig.  1,  the  current  in  coil  c  is  nil,  while  coil  c{  is 
traversed  by  its  maximum  current,  and  the  connections  may  be 
such  that  the  ring  is  magnetized  by  the  coils  ct  <?j,  as  indicated  by 
the  letters  N  s  in  Fig.  1#,  the  magnetizing  effect  of  the  coils 


12  INVENTIONS  OF  NIKOLA  TE8L A. 

c  c  being  nil,  since  these  coils  are  included   in    the   circuit  of 
coil  c. 

In  Fig.  2,  the  armature  coils  are  shown  in  a  more  advanced 
position,  one-eighth  of  one  revolution  being  completed.  Fig. 
la  illustrates  the  corresponding  magnetic  condition  of  the  ring. 
At  this  moment  the  coil  c,  generates  a  current  of  the  same  di- 


FIG.  3. 


FIG.  3a. 


rection  as  previously,  but  weaker,  producing  the  poles  wt  .Vj  upon 
the  ring ;  the  coil  c  also  generates  a  current  of  the  same  direc- 
tion, and  the  connections  may  be  such  that  the  coils  c  c  produce 
the  poles  n  *,  as  shown  in  Fig.  'la.  The  resulting  polarity  is 
indicated  by  the  letters  x  s,  and  it  will  be  observed  that  the 
poles  of  the  ring  have  been  shifted  one-eighth  of  the  periphery 
of  the  same. 

In  Fig.  3  the  armature  has  completed  one  quarter  of  one 
revolution.  In  this  phase  the  current  in  coil  c  is  a  maximum,  and 
of  such  direction  as  to  produce  the  poles  N  s  in  Fig.  3a,  whereas 
the  current  in  coil  cv  is  nil,  this  coil  being  at  its  neutral  position. 


FIG.  4. 


FIG.  4a. 


The  poles  N  s  in  Fig.  3a  are  thus  shifted  one  quarter  of  the 
circumference  of  the  ring. 

Fig.  4  shows  the  coils  c  c  in  a  still  more  advanced  position, 
the  armature  having  completed  three-eighths  of  one  revolution. 
At  that  moment  the  coil  c  still  generates  a  current  of  the  same 
direction  as  before,  but  of  less  strength,  producing  the  compar- 


POLYPHASE  CURRENTS.  13 

atively  weaker  poles  n  .y  in  Fig.  4«.  The  current  in  the  coil  Cj 
is  of  the  same  strength,  but  opposite  direction.  Its  effect  is, 
therefore,  to  produce  upon  the  ring  the  poles  n±  -^  as  indicated, 
and  a  polarity,  N  s,  results,  the  poles  now  being  shifted  three- 
eighths  of  the  periphery  of  the  ring. 

In  Fig.  5  one  half  of  one  revolution  of  the  armature  is  com- 


pleted,  and  the  resulting  magnetic  condition  of  the  ring  is  indi- 
cated in  Fig.  5«.  Now  the  current  in  coil  c  is  nil,  while  the  coil 
ct  yields  its  maximum  current,  which  is  of  the  same  direction  as 
previously ;  the  magnetizing  effect  is,  therefore,  due  to  the  coils, 
6(!  en  alone,  and,  referring  to  Fig.  5«,  it  will  be  observed  that 
the  poles  N  s  are  shifted  one  half  of  the  circumference  of  the 
ring.  During  the  next  half  revolution  the  operations  are  repeated, 
as  represented  in  the  Figs,  f>  to  8a. 

A  reference  to  the  diagrams  will  make  it  clear  that  during  one 


FIG.  Q. 


FIG.  6a. 


revolution  of  the  armature  the  poles  of  the  ring  are  shifted  once 
around  its  periphery,  and,  each  revolution  producing  like  effects, 
a  rapid  whirling  of  the  poles  in  harmony  with  the  rotation  of  the 
armature  is  the  result.  If  the  connections  of  either  one  of  the 
circuits  in  the  ring  are  reversed,  the  shifting  of  the  poles  is  made 
to  progress  in  the  opposite  direction,  but  the  operation  is  identi- 


14  INVENTIONS  OF  NIKOLA  TESLA. 

cally  the  same.  Instead  of  using  four  wires,  with  like  result) 
three  wires  may  be  used,  one  forming  a  common  return  for  both 
circuits. 

This  rotation  or  whirling  of  the  poles  manifests  itself  in  a  series 
of  curious  phenomena.  If  a  delicately  pivoted  disc  of  steel  or 
other  magnetic  metal  is  approached  to  the  ring  it  is  set  in  rapid 
rotation,  the  direction  of  rotation  varying  with  the  position  of 


FIG.  7.  FIG.  Ta. 

the  disc.  For  instance,  noting  the  direction  outside  of  the  ring 
it  will  he  found  that  inside  the  ring  it  turns  in  an  opposite  direc- 
tion, while  it  is  unaffected  if  placed  in  a  position  symmetrical  to 
the  ring.  This  is  easily  explained.  Each  time  that  a  pole  ap- 
proaches, it  induces  an  opposite  pole  in  the  nearest  point  on  the 
disc,  and  an  attraction  is  produced  upon  that  point;  owing  to  this, 
as  the  pole  is  shifted  further  away  from  the  disc  a  tangential  pull 
is  exerted  upon  the  same,  and  the  action  being  constantly  repeat- 
ed, a  more  or  less  rapid  rotation  of  the  disc  is  the  result.  As  the 
pull  is  exerted  mainly  upon  that  part  which  is  nearest  to  the 
ring,  the  rotation  outside  and  inside,  or  right  and  left,  respectively, 
is  in  opposite  directions,  Fig.  9.  When  placed  symmetrically 
to  the  ring,  the  pull  on  the  opposite  sides  of  the  disc  being  equal, 
no  rotation  results.  The  action  is  based  on  the  magnetic  inertia 
of  iron ;  for  this  reason  a  disc  of  hard  steel  is  much  more  af- 
fected than  a  disc  of  soft  iron,  the  latter  being  capable  of  very 
rapid  variations  of  magnetism.  Such  a  disc  has  proved  to  be  a 
very  useful  instrument  in  all  these  investigations,  as  it  has  en- 
abled me  to  detect  any  irregularity  in  the  action.  A  curious  ef- 
fect is  also  produced  upon  iron  tilings.  By  placing  some  upon  a 
paper  and  holding  them  externally  quite  close  to  the  ring,  they 
are  set  in  a  vibrating  motion,  remaining  in  the  same  place,  although 
the  paper  may  be  moved  back  and  forth  ;  but  in  lifting  the  paper 
to  a  certain  height  which  seems  to  be  dependent  on  the  intensity 
of  the  poles  and  the  speed  of  rotation,  they  are  thrown  away  in 


POLYPHASE  CURRENTS.  15 

a  direction  always  opposite  to  the  supposed  movement  of  the 
poles.  If  a  paper  with  filings  is  put  flat  upon  the  ring  and  the 
current  turned  on  suddenly,  the  existence  of  a  magnetic  whirl 
may  easily  be  observed. 

To  demonstrate  the  complete  analogy  between  the  ring  and  a 
revolving  magnet,  a  strongly  energized  electro-magnet  was  rota- 
ted by  mechanical  power,  and  phenomena  identical  in  every  par- 
ticular to  those  mentioned  above  were  observed. 

Obviously,  the  rotation  of  the  poles  produces  corresponding 
inductive  effects  and  may  be  utilized  to  generate  currents  in  a 
closed  conductor  placed  within  the  influence  of  the  poles.  For 
this  purpose  it  is  convenient  to  wind  a  ring  with  two  sets  of 
superimposed  coils  forming  respectively  the  primary  and  second- 
ary circuits,  as  shown  in  Fig.  10.  In  order  to  secure  the  most 
economical  results  the  magnetic  circuit  should  be  completely 
closed,  and  with  this  object  in  view  the  construction  may  be 
modified  at  will. 

The  inductive  effect  exerted  upon  the  secondary  coils  will  be 
mainly  due  to  the  shifting  or  movement  of  the  magnetic  action  ; 
but  there  may  also  be  currents  set  up  in  the  circuits  in  conse- 
quence of  the  variations  in  the  intensity  of  the  poles.  However, 
by  properly  designing  the  generator  and  determining  the  magneti- 
zing effect  of  the  primary  coils,  the  latter  element  may  be  made 
to  disappear.  The  intensity  of  the  poles  being  maintained  con- 


FIG. 


FIG.  8a. 


stant,  the  action  of  the  apparatus  will  be  perfect,  and  the  same 
result  will  be  secured  as  though  the  shifting  were  effected  by 
means  of  a  commutator  with  an  infinite  number  of  bars.  In  such 
case  the  theoretical  relation  between  the  energizing  effect  of  each 
set  of  primary  coils  and  their  resultant  magnetizing  effect  may 
be  expressed  by  the  equation  of  a  circle  having  its  centre  coin- 
ciding with  that  of  an  orthogonal  system  of  axes,  and  in  which 
the  radius  represents  the  resultant  and  the  co-ordinates  both 


16  INVENTIONS  OF  NIKOLA  TESLA. 

of  its  components.  These  are  then  respectively  the  sine  and 
cosine  of  the  angle  a  between* the  radius  and  one  of  the  axes 
(O  X\  Referring  to  Fig.  11,  we  have  ,*  =  x?  +  f ;  where 
./•  =  r  cos  a,  and  y  =  r  sin  a. 

Assuming  the  magnetizing  effect  of  each  set  of  coils  in  the 
transformer  to  be  proportional  to  the  current— which  may  be 
admitted  for  weak  degrees  of  magnetization— then  x  =  KG  and 
y  _  KC^  where  ^is  a  constant  and  c  and  c1  the  current  in  both 
sets  of  coils  respectively.  Supposing,  further,  the  field  of  the 
generator  to  be  uniform,  we  have  for  constant  speed  c1  =  A"1  sin  a 
and  c  =  Kl  sin  (90°  +  a)  =  Kl  cos  a,  where  Kl  is  a  constant. 
See  Fig.  12. 

Therefore,  a?  =  K  c  —  K  K^  cos  a; 

y  =  Kcl  =  K Kl  sin  a;  and 


FIG.  9. 

That  is,  for  a  uniform  field  the  disposition  of  the  two  coils  at 
right  angles  will  secure  the  theoretical  result,  and  the  intensity 
of  the  shifting  poles  will  be  constant.  But  from  ^  =  x?  -J-  >f  it 
follows  that  for  y  =  0,  r  =  x;  it  follows  that  the  joint  magnet- 
izing effect  of  both  sets  of  coils  should  be  equal  to  the  effect  of 
one  set  when  at  its  maximum  action.  In  transformers  and  in  a 
certain  class  of  motors  the  fluctuation  of  the  poles  is  not  of  great 
importance,  but  in  another  class  of  these  motors  it  is  desirable  to 
obtain  the  theoretical  result. 

In  applying  this  principle  to  the  construction  of  motors,  two 
typical  forms  of  motor  have  been  developed.  First,  a  form  hav- 
ing a  comparatively  small  rotary  effort  at  the  start  but  maintaining 
a  perfectly  uniform  speed  at  all  loads,  which  motor  has  been 
termed  synchronous.  Second,  a  form  possessing  a  great  rotary 
effort  at  the  start,  the  speed  being  dependent  on  the  load. 


I'OL  YMIAKK 


17 


These  motors  may  be  operated  in  three  different  ways :  1.  By 
the  alternate  currents  of  the  source  only.  2.  By  a  combined  ac- 
tion of  these  and  of  induced  currents.  3.  By  the  joint  action  of 
alternate  and  continuous  currents. 

The  simplest  form  of  a  synchronous  motor  is  obtained  by  wind- 
ing a  laminated  ring  provided  with  pole  projections  with  four 
coils,  and  connecting  the  same  in  the  manner  before  indicated. 
An  iron  disc  having  a  segment  cut  away  on  each  side  may  be  used 


Fit*  10. 

as  an  armature.  Such  a  motor  is  shown  in  Fig.  9.  The  disc 
being  arranged  to  rotate  freely  within  the  ring  in  close  proximity 
to  the  projections,  it  is  evident  that  as  the  poles  are  shifted  it 
will,  owing  to  its  tendency  to  place  itself  in  such  a  position  as  to 
embrace  the  greatest  number  of  the  lines  of  force,  closely  follow 
the  movement  of  the  poles,  and  its  motion  will  be  synchronous 
with  that  of  the  armature  of  the  generator;  that  is,  in  the  peculiar 
disposition  shown  in  Fig.  9,  in  which  the  armature  produces  by 
one  revolution  two  current  impulses  in  each  of  the  circuits.  It 
is  evident  that  if,  by  one  revolution  of  the  armature,  a  greater 
number  of  impulses  is  produced,  the  speed  of  the  motor  will  be 
correspondingly  increased.  Considering  that  the  attraction  ex- 
erted upon  the  disc  is  greatest  when  the  same  is  in  close  proximity 
to  the  poles,  it  follows  that  such  a  motor  will  maintain  exactly 
the  same  speed  at  all  loads  within  the  limits  of  its  capacity. 

To  facilitate  the  starting,  the  disc  may  be  provided  with  a  coil 
closed  upon  itself.  The  advantage  secured  by  such  a  coil  is  evi- 
dent. On  the  start  the  currents  set  up  in  the  coil  strongly  ener- 


IS 


INVENTIONS  OF  NIKOLA  TKKLA. 


gize  the  disc  and  increase  the  attraction  exerted  upon  the  same  by 
the  ring,  and  currents  being  generated  in  the  coil  as  long  as  the 
speed  of  the  armature  is  inferior  to  that  of  the  poles,  consider- 
able work  may  be  performed  by  such  a  motor  even  if  the  speed 
be  below  normal.  The  intensity  of  the  poles  being  constant,  no 
currents  will  be  generated  in  the  coil  when  the  motor  is  turning 
at  its  normal  speed. 

Instead  of  closing  the  coil  upon  itself,  its  ends  may  be  connected 
to  two  insulated  sliding  rings,  and  a  continuous  current  supplied 
to  these  from  a  suitable  generator.  The  proper  way  to  start  such 
a  motor  is  to  close  the  coil  upon  itself  until  the  normal  speed  is 
reached,  or  nearly  so,  and  then  turn  on  the  continuous  cur- 
rent. If  the  disc  be  very  strongly  energized  by  a  continuous 
current  the  motor  may  not  be  able  to  start,  but  if  it  be  weakly 
energized,  or  generally  so  that  the  magnetizing  eifect  of  the  ring 


is  preponderating,  it  will  start  and  reach  the  normal  speed.  Such 
a  motor  will  maintain  absolutely  the  same  speed  at  all  loads.  It 
has  also  been  found  that  if  the  motive  power  of  the  generator  is 
not  excessive,  by  checking  the  motor  the  speed  of  the  generator  is 
diminished  in  synchronism  with  that  of  the  motor.  It  is  charac- 
teristic of  this  form  of  motor  that  it  cannot  be  reversed  by  revers- 
ing the  continuous  current  through  the  coil. 

The  synchronism  of  these  motors  may  be  demonstrated  experi- 
mentally in  a  variety  of  ways.  For  this  purpose  it  is  best  to 
employ  a  motor  consisting  of  a  stationary  field  magnet  and  an 
armature  arranged  to  rotate  within  the  same,  as  indicated  in 
Fig.  13.  In  this  case  the  shifting  of  the  poles  of  the  armature 
produces  a  rotation  of  the  latter  in  the  opposite  direction.  It 
results  therefrom  that  when  the  normal  speed  is  readied,  the 
poles  of  the  armature  assume  fixed  positions  relatively  to  the 


POLYPHASE  CURRENTS. 


lit 


field  magnet,  and  the  same  is  magnetized  by  induction,  exhibiting 
a  distinct  pole  on  each  of  the  pole-pieces.  If  a  piece  of  soft  iron 
is  approached  to  the  field  magnet,  it  will  at  the  start  be  attracted 
with  a  rapid  vibrating  motion  produced  by  the  reversals  of  polar- 
ity of  the  magnet,  but  as  the  speed  of  the  armature  increases,  the 
vibrations  become  less  and  less  frequent  and  finally  entirely  cease. 
Then  the  iron  is  weakly  but  permanently  attracted,  showing  that 
synchronism  is  reached  and  the  field  magnet  energized  by  in- 
duction. 

The  disc  may  also  be  used  for  the  experiment.  If  held  quite 
close  to  the  armature  it  will  turn  as  long  as  the  speed  of  rotation 
of  the  poles  exceeds  that  of  the  armature ;  but  when  the  normal 


FIG.  13. 

speed  is  reached,  or  very  nearly  so,  it  ceases  to  rotate  and  is  per- 
manently attracted. 

A  crude  but  illustrative  experiment  is  made  with  an  incandes- 
cent lamp.  Placing  the  lamp  in  circuit  with  the  continuous  cur- 
rent generator  and  in  series  with  the  magnet  coil,  rapid  fluctua- 
tions are  observed  in  the  light  in  consequence  of  the  induced  cur- 
rents set  up  in  the  coil  at  the  start ;  the  speed  increasing,  the 
fluctuations  occur  at  longer  intervals,  until  they  entirely  disap- 
pear, showing  that  the  motor  has  attained  its  normal  speed.  A 
telephone  receiver  affords  a  most  sensitive  instrument ;  when 
connected  to  any  circuit  in  the  motor  the  synchronism  may  be 
easily  detected  on  the  disappearance  of  the  induced  currents. 

In  motors  of  the  synchronous  type  it  is  desirable  to  maintain 


20 


INVENTIONS  OF  NIKOLA   TKSLA. 


the  quantity  of  the  shifting  magnetism  constant,  especially  if  the 
magnets  are  not  properly  subdivided. 

To  obtain  a  rotary  effort  in  these  motors  was  the  subject  of 
long  thought.  In  order  to  secure  this  result  it  was  necessary  to 
make  such  a  disposition  that  while  the  poles  of  one  element  of 
the  motor  are  shifted  by  the  alternate  currents  of  the  source,  the 
poles  produced  upon  the  other  elements  should  always  be  main- 
tained in  the  proper  relation  to  the  former,  irrespective  of  the 
speed  of  the  motor.  Such  a  condition  exists  in  a  continuous 
current  motor ;  but  in  a  synchronous  motor,  such  as  described, 
this  condition  is  fulfilled  only  when  the  speed  is  normal. 

The  object  has  been  attained  by  placing  within  the  ring  a  prop- 
erly subdivided  cylindrical  iron  core  wound  with  several  indepen- 
dent coils  closed  upon  themselves.  Two  coils  at  right  angles  as 


6 


FIG.  14. 


in  Fig.  14,  are  sufficient,  but  a  greater  number  may  be  advan- 
tageously employed.  It  results  from  this  disposition  that  when 
the  poles  of  the  ring  are  shifted,  currents  are  generated  in  the 
closed  armature  coils.  These  currents  are  the  most  intense  at  or 
near  the  points  of  the  greatest  density  of  the  lines  of  force,  and 
their  effect  is  to  produce  poles  upon  the  armature  at  right  angles 
to  those  of  the  ring,  at  least  theoretically  so ;  and  since  this  action 
is  entirely  independent  of  the  speed — that  is,  as  far  as  the  location 
of  the  poles  is  concerned — a  continuous  pull  is  exerted  upon  the 
periphery  of  the  armature.  In  many  respects  these  motors  are 
similar  to  the  continuous  current  motors.  If  load  is  put  on,  the 
speed,  and  also  the  resistance  of  the  motor,  is  diminished  and 
more  current  is  made  to  pass  through  the  energizing  coils,  thus 


POLYPHASE  CURRENTS.  21 

increasing  the  effort.  Upon  the  load  being  taken  off,  the 
counter-electromotive  force  increases  and  less  current  passes 
through  the  primary  or  energizing  coils.  Without  any  load  the 
speed  is  very  nearly  equal  to  that  of  the  shifting  poles  of  the 
iield  magnet. 

It  will  be  found  that  the  rotary  effort  in  these  motors  fully 


FIG.  15.  FIG.  16.  FIG.  17. 

equals  that  of  the  continuous  current  motors.  The  effort  seems 
to  be  greatest  when  both  armature  and  field  magnet  are  without 
any  projections  ;  but  as  in  such  dispositions  the  field  cannot  be 
concentrated,  probably  the  best  results  will  be  obtained  by  leav- 
ing pole  projections  on  one  of  the  elements  only.  Generally,  it 
may  be  stated  the  projections  diminish  the  torque  and  produce  a 
tendency  to  synchronism. 

A  characteristic  feature  of  motors  of  this  kind  is  their  property 
of  being  very  rapidly  reversed.  This  follows  from  the  peculiar 
action  of  the  motor.  Suppose  the  armature  to  be  rotating  and 
the  direction  of  rotation  of  the  poles  to  be  reversed.  The  appa- 
ratus then  represents  a  dynamo  machine,  the  power  to  drive  this 
machine  being  the  momentum  stored  up  in  the  armature  and  its 
speed  being  the  sum  of  the  speeds  of  the  armature  and  the 
poles. 

If  we  now  consider  that  the  power  to  drive  such  a  dynamo 


'\AAA/ 


FIG.  18.  FIG.  19.          FIG.  20.  FIG.  21. 

would  be  very  nearly  proportional  to  the  third  power  of  the 
speed,  for  that  reason  alone  the  armature  should  be  quickly  re- 
versed. But  simultaneously  with  the  reversal  another  element  is 
brought  into  action,  namely,  as  the  movement  of  the  poles  with 
respect  to  the  armature  is  reversed,  the  motor  acts  like  a  trans- 
former in  which  the  resistance  of  the  secondarv  circuit  would  be 


gg  INVENTIONS  OF  NIKOLA  TE8LA. 

abnormally  diminished  by  producing  in  this  circuit  an  additional 
electromotive  force.  Owing  to  these  causes  the  reversal  is  in- 
stantaneous. 

If  it  is  desirable  to  secure  a  constant  speed,  and  at  the  same 
time  a  certain  effort  at  the  start,  this  result  may  be  easily  attained 
in  a  variety  of  ways.  For  instance,  two  armatures,  one  for  torque 
and  the  other  for  synchronism,  may  be  fastened  on  the  same  shaft 
and  any  desired  preponderance  may  be  given  to  either  one,  or  an 
armature  may  be  wound  for  rotary  effort,  but  a  more  or  less  pro- 
nounced tendency  to  synchronism  may  be  given  to  it  by  properly 
constructing  the  iron  core ;  and  in  many  other  ways. 

As  a  means  of  obtaining  the  required  phase  of  the  currents  in 
both  the  circuits,  the  disposition  of  the  two  coils  at  right  angles 
is  the  simplest,  securing  the  most  uniform  action  ;  but  the  phase 
may  be  obtained  in  many  other  ways,  varying  with  the  machine 
employed.  Any  of  the  dynamos  at  present  in  use  may  be  easily 
adapted  for  this  purpose  by  making  connections  to  proper  points 
of  the  generating  coils.  In  closed  circuit  armatures,  such  as  used 
in  the  continuous  current  systems,  it  is  best  to  make  four  deriva- 
tions from  equi-distant  points  or  bars  of  the  commutator,  and  to 
connect  the  same  to  four  insulated  sliding  rings  on  the  shaft.  In 
this  case  each  of  the  motor  circuits  is  connected  to  two  diametri- 
cally opposite  bars  of  the  commutator.  In  such  a  disposition  the 
motor  may  also  be  operated  at  half  the  potential  and  on  the  three- 
wire  plan,  by  connecting  the  motor  circuits  in  the  proper  order  to 
three  of  the  contact  rings. 

In  multipolar  dynamo  machines,  such  as  used  in  the  converter 
systems,  the  phase  is  conveniently  obtained  by  winding  upon  the 
armature  two  series  of  coils  in  such  a  manner  that  while  the  coils 
of  one  set  or  series  are  at  their  maximum  production  of  current, 
the  coils  of  the  other  will  be  at  their  neutral  position,  or  nearly 
so,  whereby  both  sets  of  coils  may  be  subjected  simultaneously 
or  successively  to  the  inducing  action  of  the  field  magnets. 

Generally  the  circuits  in  the  motor  will  be  similarly  disposed, 
and  various  arrangements  may  be  made  to  fulfill  the  requirements; 
but  the  simplest  and  most  practicable  is  to  arrange  primary  cir- 
cuits on  stationary  parts  of  the  motor,  thereby  obviating,  at  least 
in  certain  forms,  the  employment  of  sliding  contacts.  In  such  a 
case  the  magnet  coils  are  connected  alternately  in  both  the  cir- 
cuits ;  that  is,  1,  3,  5 in  one,  and  2,  4,  6 in  the  other,  and 

the  coils  of  each  set  of  series  may  be  connected  all  in  the  same 


POLYPHASE  CURRENTS.  23 

manner,  or  alternately  in  opposition ;  in  the  latter  case  a  motor 
with  half  the  number  of  poles  will  result,  and  its  action  will  be 
correspondingly  modified.  The  Figs.  15,  16,  and  17,  show 
three  different  phases,  the  magnet  coils  in  each  circuit  being  con- 
nected alternately  in  opposition.  In  this  case  there  will  be  always 
four  poles,  as  in  Figs.  15  and  17 ;  four  pole  projections  will  be 
neutral ;  and  in  Fig.  16  two  adjacent  pole  projections  will  have 
the  same  polarity.  If  the  coils  are  connected  in  the  same  manner 
there  will  be  eight  alternating  poles,  as  indicated  by  the  letters 
n'  s'  in  Fig.  15. 

The  employment  of  multipolar  motors  secures  in  this  system  an 
advantage  much  desired  and  unattainable  in  the  continuous  cur- 
rent system,  and  that  is,  that  a  motor  may  be  made  to  run  exactly 
at  a  predetermined  speed  irrespective  of  imperfections  in  con- 
struction, of  the  load,  and,  within  certain  limits,  of  electromotive 
force  and  current  strength. 

In  a  general  distribution  system  of  this  kind  the  following  plan 
should  be  adopted.  At  the  central  station  of  supply  a  generator 
should  be  provided  having  a  considerable  number  of  poles.  The 
motors  operated  from  this  generator  should  be  of  the  synchronous 
type,  but  possessing  sufficient  rotary  effort  to  insure  their  starting. 
With  the  observance  of  proper  rules  of  construction  it  may  be 
admitted  that  the  speed  of  each  motor  will  be  in  some  inverse 
proportion  to  its  size,  and  the  number  of  poles  should  be  chosen 
accordingly.  Still,  exceptional  demands  may  modify  this  rule. 
In  view  of  this,  it  will  be  advantageous  to  provide  each  motor 
with  a  greater  number  of  pole  projections  or  coils,  the  number 
being  preferably  a  multiple  of  two  and  three.  By  this  means,  by 
simply  changing  the  connections  of  the  coils,  the  motor  may  be 
adapted  to  any  probable  demands. 

If  the  number  of  the  poles  in  the  motor  is  even,  the  action  will 
be  harmonious  and  the  proper  result  will  be  obtained ;  if  this 
is  not  the  case,  the  best  plan  to  be  followed  is  to  make  a 
motor  with  a  double  number  of  poles  and  connect  the  same  in 
the  manner  before  indicated,  so  that  half  the  number  of  poles 
result.  Suppose,  for  instance,  that  the  generator  has  twelve  poles, 
and  it  would  be  desired  to  obtain  a  speed  equal  to  ^  of  the  speed 
of  the  generator.  This  would  require  a  motor  with  seven  pole 
projections  or  magnets,  and  such  a  motor  could  not  be  properly 
connected  in  the  circuits  unless  fourteen  armature  coils  would  be 
provided,  which  would  necessitate  the  employment  of  sliding 


£4  INVENTIONS  OF  NIKOLA  TESLA. 

contacts.  To  avoid  this,  the  motor  should  be  provided  with  four- 
teen magnets  and  seven  connected  in  each  circuit,  the  magnets 
in  each  circuit  alternating  among  themselves.  The  armature 
should  have  fourteen  closed  coils.  The  action  of  the  motor  will 
not  be  quite  as  perfect  as  in  the  case  of  an  even  number  of  poles, 
but  the  drawback  will  not  be  of  a  serious  nature. 

However,  the  disadvantages  resulting  from  this  unsymmetrical 
form  will  be  reduced  in  the  same  proportion  as  the  number  of 
the  poles  is  augmented. 

If  the  generator  has,  say,  n,  and  the  motor  %  poles,  the  speed 
of  the  motor  will  be  equal  to  that  of  the  generator  multiplied  by 


The  speed  of  the  motor  will  generally  be  dependent  on  the 
number  of  the  poles,  but  there  may  be  exceptions  to  this  rule. 
The  speed  may  be  modified  by  the  phase  of  the  currents  in  the 
circuit  or  by  the  character  of  the  current  impulses  or  by  inter- 
vals between  each  or  between  groups  of  impulses.  Some  of  the 
possible  cases  are  indicated  in  the  diagrams,  Figs.  18,  19,  20  and 
21,  which  are  self-explanatory.  Fig.  18  represents  the  condi- 
tion generally  existing,  and  which  secures  the  best  result.  In 
such  a  case,  if  the  typical  form  of  motor  illustrated  in  Fig.  9 
is  employed,  one  complete  wave  in  each  circuit  will  produce  one 
revolution  of  the  motor.  In  Fig.  19  the  same  resiilt  will  be 
effected  by  one  wave  in  each  circuit,  the  impulses  being  succes- 
sive; in  Fig.  20  by  four,  and  in  Fig.  21  by  eight  waves. 

By  such  means  any  desired  speed  may  be  attained,  that  is,  at 
least  within  the  limits  of  practical  demands.  This  system  pos- 
sesses this  advantage,  besides  others,  resulting  from  simplicity. 
At  full  loads  the  motors  show  an  efficiency  fully  equal  to  that  of 
the  continuous  current  motors.  The  transformers  present  an 
additional  advantage  in  their  capability  of  operating  motors. 
They  are  capable  of  similar  modifications  in  construction,  and  will 
facilitate  the  introduction  of  motors  and  their  adaptation  to  prac- 
tical demands.  Their  efficiency  should  be  higher  than  that  of 
the  present  transformers,  and  I  base  my  assertion  on  the  fol- 
lowing : 

In  a  transformer,  as  constructed  at  present,  we  produce  the 
currents  in  the  secondary  circuit  by  varying  the  strength  of  the 
primary  or  exciting  currents.  If  we  admit  proportionality  with 
respect  to  the  iron  core  the  inductive  effect  exerted  upon  the 


POLYPHASE  CURRENTS.  25 

secondary  coil  will  be  proportional  to  the  numerical  sum  of  the 
variations  in  the  strength  of  the  exciting  current  per  unit  of  time; 
whence  it  follows  that  for  a  given  variation  any  prolongation  of 
the  primary  current  will  result  in  a  proportional  loss.  In  order 
to  obtain  rapid  variations  in  the  strength  of  the  current,  essential 
to  efficient  induction,  a  great  number  of  undulations  are  employ- 
ed ;  from  this  practice  various  disadvantages  result.  These  are  : 
Increased  cost  and  diminished  efficiency  of  the  generator ;  more 
waste  of  energy  in  heating  the  cores,  and  also  diminished  output 
of  the  transformer,  since  the  core  is  not  properly  utilized,  the 
reversals  being  too  rapid.  The  inductive  effect  is  also  very  small 
in  certain  phases,  as  will  be  apparent  from  a  graphic  representa- 
tion, and  there  may  be  periods  of  inaction,  if  there  are  intervals 
between  the  succeeding  current  impulses  or  waves.  In  producing 
a  shifting  of  the  poles  in  a  transformer,  and  thereby  inducing 
currents,  the  induction  is  of  the  ideal  character,  being  always 
maintained  at  its  maximum  action.  It  is  also  reasonable  to  as- 
sume that  by  a  shifting  of  the  poles  less  energy  will  be  wasted 
than  by  reversals. 


CHAPTER  IV. 

MODIFICATIONS  AND  EXPANSIONS  or  THE  TESLA  POLYPHASE 

SYSTEMS. 

IN  his  earlier  papers  and  patents  relative  to  polyphase  currents, 
Mr.  Tesla  devoted  himself  chiefly  to  an  enunciation  of  the  broad 
lines  and  ideas  lying  at  the  basis  of  this  new  work ;  but  he  sup- 
plemented this  immediately  by  a  series  of  other  striking  inven- 
tions which  may  be  regarded  as  modifications  and  expansions  of 
certain  features  of  the  Tesla  systems.  These  we  shall  now  pro- 
ceed to  deal  with. 

In  the  preceding  chapters  we  have  thus  shown  and  described 
the  Tesla  electrical  systems  for  the  transmission  of  power  and  the 
conversion  and  distribution  of  electrical  energy,  in  which  the 
motors  and  the  transformers  contain  two  or  more  coils  or  sets  of 
coils,  which  were  connected  up  in  independent  circuits  with 
corresponding  coils  of  an  alternating  current  generator,  the  opera- 
tion of  the  system  being  brought  about  by  the  co-operation  of 
the  alternating  currents  in  the  independent  circuits  in  progres- 
sively moving  or  shifting  the  poles  or  points  of  maximum  mag- 
netic effect  of  the  motors  or  converters.  In  these  systems  two 
independent  conductors  are  employed  for  each  of  the  independ- 
ent circuits  connecting  the  generator  with  the  devices  for  con- 
verting the  transmitted  currents  into  mechanical  energy  or  into 
electric  currents  of  another  character.  This,  however,  is  not 
always  necessary.  The  two  or  more  circuits  may  have  a  single 
return  path  or  wire  in  common,  with  a  loss,  if  any,  which  is  so 
extremely  slight  that  it  may  be  disregarded  entirely.  For  the 
sake  of  illustration,  if  the  generator  have  two  independent  coils 
and  the  motor  two  coils  or  two  sets  of  coils  in  corresponding  Vela- 
tions  to  its  operative  elements  one  terminal  of  each  generator 
coil  is  connected  to  the  corresponding  terminals  of  the  motor 
coils  through  two  independent  conductors,  while  the  opposite 
terminals  of  the  respective  coils  are  both  connected  to  one 
return  wire.  The  following  description  deals  with  the  modifica- 


POLYPHASE  CURRENTS. 


27 


tion.  Fig.  22  is  a  diagrammatic  illustration  of  a  generator  and 
single  motor  constructed  and  electrically  connected  in  accord- 
ance with  the  invention.  Fig.  23  is  a  diagram  of  the  system 
as  it  is  nsed  in  operating  motors  or  converters,  or  both,  in  parallel, 
while  Fig.  24  illustrates  diagrammatically  the  manner  of  operat- 
ing two  or  more  motors  or  converters,  or  both,  in  series.  Refer- 
ring to  Fig.  22,  A  A  designate  the  poles  of  the  field  magnets  of 
an  alternating-current  generator,  the  armature  of  which,  being  in 
this  case  cylindrical  in  form  and  mounted  on  a  shaft,  c,  is  wound 


FIG.  24. 


longitudinally  with  coils  B  B'.  The  shaft  c  carries  three  insulated 
contact-rings,  a  b  c,  to  two  of  which,  as  5  c,  one  terminal  of  each 
coil,  as  e  d,  is  connected.  The  remaining  terminals,  f  g,  are  both 
connected  to  the  third  ring,  a. 

A  motor  in  this  case  is  shown  as  composed  of  a  ring,  H,  wound 
with  four  coils,  i  i  j  j,  electrically  connected,  so  as  to  co-operate 
in  pairs,  with  a  tendency  to  fix  the  poles  of  the  ring  at  four  points 
ninety  degrees  apart.  Within  the  magnetic  ring  H  is  a  disc  or 
cylindrical  core  wound  with  two  coils,  G  a',  which  may  be  con- 


28 


INVENTIONS  OF  NIKOLA  TESLA. 


nected  to  form  two  closed  circuits.  The  terminals  j  k  of  the  two 
sets  or  pairs  of  coils  are  connected,  respectively,  to  the  binding- 
posts  E'  F',  and  the  other  terminals,  h  i,  are  connected  to  a  single 
binding-post,  D'.  To  operate  the  motor,  three  line-wires  are  used 
to  connect  the  terminals  of  the  generator  with  those  of  the  mo- 
tor. 

So  far  as  the  apparent  action  or  mode  of  operation  of  this  ar- 
rangement is  concerned,  the  single  wire  D,  which  is,  so  to  speak, 


FIG.  23. 

a  common  return-wire  for  both  circuits,  may  be  regarded  as  two 
independent  wires.  In  the  illustration,  with  the  order  of  con- 
nection shown,  coil  B'  of  the  generator  is  producing  its  maximum 
current  and  coil  B  its  minimum ;  hence  the  current  which  passes 
through  wire  e,  ring  5,  brush  b' ',  line-wire  E,  terminal  E',  wire,;', 
coils  i  i,  wire  or  terminal  D',  line-wire  D,  brush  a',  ring  a,  and 
wire/,  fixes  the  polar  line  of  the  motor  midway  between  the 


POLYPHASE  VURRKNT8.  25) 

two  coils  i  i ;  but  as  the  coil  B'  moves  from  the  position  indicated 
it  generates  less  current,  while  coil  B,  moving  into  the  field,  gen- 
erates more.  The  current  from  coil  B  passes  through  the  devices 
and  wires  designated  by  the  letters  d,  c,  c'  F,  F'  &,  j  j,  i,  D',  D,  #', 
«,  and  g,  and  the  position  of  the  poles  of  the  motor  will  be  due 
to  the  resultant  effect  of  the  currents  in  the  two  sets  of  coils — 
that  is,  it  will  be  advanced  in  proportion  to  the  advance  or  for- 
ward movement  of  the  armature  coils.  The  movement  of  the 
generator-armature  through  one-quarter  of  a  revolution  will  ob- 
viously bring  coil  B'  into  its  neutral  position  and  coil  B  into  its 
position  of  maximum  effect,  and  this  shifts  the  poles  ninety  de- 
grees, as  they  are  fixed  solely  by  coils  B.  This  action  is  repeated 
for  each  quarter  of  a  complete  revolution. 

When  more  than  one  motor  or  other  device  is  employed,  they 
may  be  run  either  in  parallel  or  series.  In  Fig.  23  the  former 
arrangement  is  shown.  The  electrical  device  is  shown  as  a  con- 
verter, L,  of  which  the  two  sets  of  primary  coils  p  r  are  con- 
nected, respectively,  to  the  mains  F  E,  which  are  electrically  con- 
nected with  the  two  coils  of  the  generator.  The  cross-circuit 
wires  I  m,  making  these  connections,  are  then  connected  to  the 
common  return-wire  D.  The  secondary  coils  p'  p"  are  in  circuits 
n  <>,  including,  for  example,  incandescent  lamps.  Only  one  con- 
verter is  shown  entire  in  this  figure,  the  others  being  illustrated 
diagrammatically. 

When  motors  or  converters  are  to  be  run  in  series,  the  two 
wires  E  F  are  led  from  the  generator  to  the  coils  of  the  first 
motor  or  converter,  then  continued  on  to  the  next,  and  so  on 
through  the  whole  series,  and  are  then  joined  to  the  single  wire 
D,  which  completes  both  circuits  through  the  generator.  This  is 
shown  in  Fig.  24,  in  which  j  i  represent  the  two  coils  or  sets  of 
coils  of  the  motors. 

There  are,  of  course,  other  conditions  under  which  the  same 
idea  may  be  carried  out.  For  example,  in  case  the  motor  and 
generator  each  has  three  independent  circuits,  one  terminal  of 
each  circuit  is  connected  to  a  line-wire,  and  the  other  three  ter- 
minals to  a  common  return-conductor.  This  arrangement  will 
secure  similar  results  to  those  attained  with  a  generator  and  motor 
having  but  two  independent  circuits,  as  above  described.- 

When  applied  to  such  machines  and  motors  as  have  three  or 
more  induced  circuits  with  a  common  electrical  joint,  the  three 
or  more  terminals  of  the  generator  would  be  simply  connected 


30  INVENTIONS  OF  NIKOLA  TESLA. 

to  those  of  the  motor.  Mr.  Tesla  states,  however,  that  the  re- 
sults obtained  in  this  manner  show  a  lower  efficiency  than  do  the 
forms  dwelt  upon  more  fully  above. 


CHAPTER  V. 

UTILIZING  FAMILIAR  TYPES  OF  GENERATOR  OF  THE  CONTINUOUS 
CURRENT  TYPE. 

THE  preceding  descriptions  have  assumed  the  use  of  alternating 
current  generators  in  which,  in  order  to  produce  the  progressive 
movement  of  the  magnetic  poles,  or  of  the  resultant  attraction  of 
independent  field  magnets,  the  current  generating  coils  are  inde- 
pendent or  separate.  The  ordinary  forms  of  continuous  current 
dynamos  may,  however,  be  employed  for  the  same  work,  in 
accordance  with  a  method  of  adaptation  devised  by  Mr.  Tesla. 
As  will  be  seen,  the  modification  involves  but  slight  changes  in 
their  construction,  and  presents  other  elements  of  economy. 

On  the  shaft  of  a  given  generator,  either  in  place  of  or  in  ad- 
dition to  the  regular  commutator,  are  secured  as  many  pairs  of 
insulated  collecting-rings  as  there  are  circuits  to  be  operated. 
Now,  it  will  be  understood  that  in  the  operation  of  any  dynamo 
electric  generator  the  currents  in  the  coils  in  their  movement 
through  the  field  of  force  undergo  different  phases — that  is  to 
say,  at  different  positions  of  the  coils  the  currents  have  certain 
directions  and  certain  strengths— and  that  in  the  Tesla  motors  or 
transformers  it  is  necessary  that  the  currents  in  the  energizing 
coils  should  undergo  a  certain  order  of  variations  in  strength  and 
direction.  Hence,  the  further  step — viz.,  the  connection  between 
the  induced  or  generating  coils  of  the  machine  and  the  contact- 
rings  from  which  the  currents  are  to  be  taken  off — will  be  deter- 
mined solely  by  what  order  of  variations  of  strength  and  direction 
in  the  currents  is  desired  for  producing  a  given  result  in  the 
electrical  translating  device.  This  may  be  accomplished  in 
various  ways ;  but  in  the  drawings  we  give  typical  instances  only 
of  the  best  and  most  practicable  ways  of  applying  the  invention 
to  three  of  the  leading  types  of  machines  in  widespread  use,  in 
order  to  illustrate  the  principle. 

Fig.  25  is  a  diagram  illustrative  of  the  mode  of  applying  the 
invention  to  the  well-known  type  of  "  closed  "  or  continuous  cir- 


32  INVENTIONS  OF  NIKOLA  TESLA. 

cuit  machines.  Fig.  26  is  a  similar  diagram  embodying  an  arma- 
ture with  separate  coils  connected  diametrically,  or  what  is  gener- 
ally called  an  "open-circuit"  machine.  Fig.  27  is  a  diagram 
showing  the  application  of  the  invention  to  a  machine  the  arm- 
ature-coils of  which  have  -a  common  joint. 

Keferring  to  Fig.  25,  let  A  represent  a  Tesla  motor  or  trans- 
former which,  for  convenience,  we  will  designate  as  a  "con- 
verter." It  consists  of  an  annular  core,  B,  wound  with  four  inde- 
pendent coils,  c  and  D,  those  diametrically  opposite  being  con- 


FIG.  25. 

nected  together  so  as  to  co-operate  in  pairs  in  establishing  free 
poles  in  the  ring,  the  tendency  of  each  pair  being  to  fix  the  poles 
at  ninety  degrees  from  the  other.  There  may  be  an  armature, 
E,  within  the  ring,  which  is  wound  with  coils  closed  upon  them- 
selves. The  object  is  to  pass  through  coils  c  D  currents  of  such 
relative  strength  and  direction  as  to  produce  a  progressive  shift- 
ing or  movement  of  the  points  of  maximum  magnetic  effect 
around  the  ring,  and  to  thereby  maintain  a  rotary  movement  of 
the  armature.  There  are  therefore  secured  to  the  shaft  F  of  the 
generator,  four  insulated  contact-rings,  abed,  upon  which  bear 


POLYPHASE  CURRENTS.  33 

the  collecting-brushes  a'  b'  c'  d',  connected  by  wires  G  G  H  H,  re- 
spectively, with  the  terminals  of  coils  c  and  D. 

Assume,  for  sake  of  illustration,  that  the  coils  D  D  are  to  re- 
ceive the  maximum  and  coils  c  c  at  the  same  instant  the  mini- 
mum current,  so  that  the  polar  line  may  be  midway  between  the 
coils  D  D.  The  rings  a  5  would  therefore  be  connected  to  the 
continuous  armature-coil  at  its  neutral  points  with  respect  to  the 
field,  or  the  point  corresponding  with  that  of  the  ordinary  com- 
mutator brushes,  and  between  which  exists  the  greatest  differ- 
ence of  potential ;  while  rings  c  d  would  be  connected  to  two 
points  in  the  coil,  between  which  exists  no  difference  of  potential. 
The  best  results  will  be  obtained  by  making  these  connections  at 
points  equidistant  from  one  another,  as  shown.  These  connec- 
tions are  easiest  made  by  using  wires  L  between  the  rings  and  the 
loops  or  wires  j,  connecting  the  coil  i  to  the  segments  of  the 
commutator  K.  When  the  converters  are  made  in  this  manner, 
it  is  evident  that  the  phases  of  the  currents  in  the  sections  of  the 
generator  coil  will  be  reproduced  in  the  converter  coils.  For 
example,  after  turning  through  an  arc  of  ninety  degrees  the  con- 
ductors L  L,  which  before  conveyed  the  maximum  current,  will 
receive  the  minimum  current  by  reason  of  the  change  in  the 
position  of  their  coils,  and  it  is  evident  that  for  the  same  reason 
the  current  in  these  coils  lias  gradually  fallen  from  the  maximum 
to  the  minimum  in  passing  through  the  arc  of  ninety  degrees. 
In  this  special  plan  of  connections,  the  rotation  of  the  magnetic 
poles  of  the  converter  will  be  synchronous  with  that  of  the 
armature  coils  of  the  generator,  and  the  result  will  be  the  same, 
whether  the  energizing  circuits  are  derivations  from  a  continuous 
armature  coil  or  from  independent  coils,  as  in  Mr.  Tesla's 
other  devices. 

In  Fig.  25,  the  brushes  M  M  are  shown  in  dotted  lines  in  their 
proper  normal  position.  In  practice  these  brushes  may  be  re- 
moved from  the  commutator  and  the  field  of  the  generator 
excited  by  an  external  source  of  current;  or  the  brushes  may  be 
allowed  to  remain  on  the  commutator  and  to  take  off  a  converted 
current  to  excite  the  field,  or  to  be  used  for  other  purposes. 

In  a  certain  well-known  class  of  machines  known  as  the  "open 
circuit,"  the  armature  contains  a  number  of  coils  the  terminals  of 
which  connect  to  commutator  segments,  the  coils  being  connected 
across  the  armature  in  pairs.  This  type  of  machine  is  repre- 
sented in  Fig.  2fi.  In  this  machine  each  pair  of  coils  goes 


34  INVENTIONS  OF  NIKOLA  TESLA. 

through  the  same  phases  as  the  coils  in  some  of  the  generators 
already  shown,  and  it  is  obviously  only  necessary  to  utilize  them 
in  pairs  or  sets  to  operate  a  Tesla  converter  by  extending  the 
segments  of  the  commutators  belonging  to  each  pair  of  coils  and 
causing  a  collecting  brush  to  bear  on  the  continuous  portion  of 
each  segment.  In  this  way  two  or  more  circuits  may  be  taken 
off  from  the  generator,  each  including  one  or  more  pairs  or  sets 
of  coils  as  may  be  desired. 

In  Fig.  2H  i  i  represent  the  armature  coils,  T  T  the  poles  of  the 
field  magnet,  and  F  the  shaft  carrying  the  commutators,  which 
are  extended  to  form  continuous  portions  a  I  c  d.  The  brushes 


FIG.  26. 


FIG.  27. 


bearing  on  the  continuous  portions  for  taking  off  the  alternating 
currents  are  represented  by  a'  V  c'  d'.  The  collecting  brushes, 
or  those  which  may  be  used  to  take  off  the  direct  current,  are 
designated  by  M  M.  Two  pairs  of  the  armature  coils  and  their 
commutators  are  shown  in  the  figure  as  being  utilized;  but  all 
may  be  utilized  in  a  similar  manner. 

There  is  another  well-known  type  of  machine  in  which  three 
or  more  coils,  A'  «'  c',  on  the  armature  have  a  common  joint, 
the  free  ends  being  connected  to  the  segments  of  a  commutator. 
This  form  of  generator  is  illustrated  in  Fig.  27.  In  this  case  each 
terminal  of  the  generator  is  connected  directly  or  in  derivation 
to  a  continuous  ring,  a  1)  <?,  and  collecting  brushes,  a'  V  c',  bearing 


POLYPHASE  CURRENT*.  33 

thereon,  take  oft'  the  alternating  currents  that  operate  the  motor. 
It  is  preferable  in  this  case  to  employ  a  motor  or  transformer 
with  three  energizing  coils,  A"  B"  c",  placed  symmetrically  with 
those  of  the  generator,  and  the  circuits  from  the  latter  are  con- 
nected to  the  terminals  of  such  coils  either  directly — as  when 
they  are  stationary — or  by  means  of  brushes  e'  and  contact  rings 
e.  In  this,  as  in  the  other  cases,  the  ordinary  commutator  may 
be  used  on  the  generator,  and  the  current  taken  from  it  utilized 
for  exciting  the  generator  iielcl-magnets  or  for  other  purposes. 


CHAPTER  VI. 

METHOD  OF  OBTAINING  DESIRED  SPEED  OF  MOTOR  OR 
GENERATOR. 

WITH  the  object  of  obtaining  the  desired  speed  in  motors 
operated  by  means  of  alternating  currents  of  differing  phase, 
Mr.  Tesla  has  devised  various  plans  intended  to  meet  the  prac- 
tical requirements  of  the  case,  in  adapting  his  system  to  types  of 
multipolar  alternating  current  machines  yielding  a  large  number 
of  current  reversals  for  each  revolution. 

For  example,  Mr.  Tesla  has  pointed  out  that  to  adapt  a  given 
type  of  alternating  current  generator,  you  may  couple  rigidly 
two  complete  machines,  securing  them  together  in  such  a  way 
that  the  requisite  difference  in  phase  will  be  produced ;  or  you 
may  fasten  two  armatures  to  the  same  shaft  within  the  influence 
of  the  same  field  and  with  the  requisite  angular  displacement  to 
yield  the  proper  difference  in  phase  between  the  two  currents; 
or  two  armatures  may  be  attached  to  the  same  shaft  with  their 
coils  symmetrically  disposed,  but  subject  to  the  influence  of  two 
sets  of  field  magnets  duly  displaced ;  or  the  two  sets  of  coils 
may  be  wound  on  the  same  armature  alternately  or  in  such  man- 
ner that  they  will  develop  currents  the  phases  of  which  differ  in 
time  sufficiently  to  produce  the  rotation  of  the  motor. 

Another  method  included  in  the  scope  of  the  same  idea,  where- 
by a  single  generator  may  run  a  number  of  motors  either  at  its 
own  rate  of  speed  or  all  at  different  speeds,  is  to  construct  the 
motors  with  fewer  poles  than  the  generator,  in  which  case  their 
speed  will  be  greater  than  that  of  the  generator,  the  rate  of  speed 
being  higher  as  the  number  of  their  poles  is  relatively  less.  This 
may  be  understood  from  an  example,  taking  a  generator  that  has 
two  independent  generating  coils  which  revolve  between  two 
pole  pieces  oppositely  magnetized ;  and  a  motor  with  energizing 
coils  that  produce  at  any  given  time  two  magnetic  poles  in  one 
element  that  tend  to  set  up  a  rotation  of  the  motor.  A  genera- 
tor thus  constructed  yields  four  reversals,  or  impulses,  in  each 


POLYPHASE  CURRENTS.  37 

revolution,  two  in  each  of  its  independent  circuits ;  and  the  effect 
upon  the  motor  is  to  shift  the  magnetic  poles  through  three  hun- 
dred and  sixty  degrees.  It  is  obvious  that  if  the  four  reversals 
in  the  same  order  could  be  produced  by  each  half-revolution  of 
the  generator  the  motor  would  make  two  revolutions  to  the  gen- 
erator's one.  This  would  be  readily  accomplished  by  adding  two 
intermediate  poles  to  the  generator  or  altering  it  in  any  of  the 
other  equivalent  ways  above  indicated.  The  same  rule  applies 
to  generators  and  motors  with  multiple  poles.  For  instance,  if  a 
generator  be  constructed  with  two  circuits,  each  of  which  pro- 
duces twelve  reversals  of  current  to  a  revolution,  and  these  cur- 
rents be  directed  through  the  independent  energizing-coils  of  a 
motor,  the  coils  of  which  are  so  applied  as  to  produce  twelve 


FIG.  28,  FIG.  29. 

magnetic  poles  at  all  times,  the  rotation  of  the  two  will  be  syn- 
chronous ;  but  if  the  motor-coils  produce  but  six  poles,  the  movable 
element  will  be  rotated  twice  while  the  generator  rotates  once ;  or 
if  the  motor  have  four  poles,  its  rotation  will  be  three  times  as 
fast  as  that  of  the  generator. 

These  features,  so  far  as  necessary  to  an  understanding  of  the 
principle,  are  here  illustrated.  Fig.  28  is  a  diagrammatic  illus- 
tration of  a  generator  constructed  in  accordance  with  the  inven- 
tion. Fig.  29  is  a  similar  view  of  a  correspondingly  constructed 
motor.  Fig.  30  is  a  diagram  of  a  generator  of  modified  con- 
struction. Fig.  31  is  a  diagram  of  a  motor  of  corresponding 
character.  Fig.  32  is  a  diagram  of  a  system  containing  a  gener- 
ator and  several  motors  adapted  to  run  at  various  speeds. 


38  INVENTIONS  OF  NIKOLA  TESLA. 

In  Fig.  28,  let  c  represent  a  cylindrical  armature  core  wound 
longitudinally  with  insulated  coils  A  A,  which  are  connected  up 
in  series,  the  terminals  of  the  series  being  connected  to  collecting- 
rings  a  a  on  the  shaft  G.  By  means  of  this  shaft  the  armature 
is  mounted  to  rotate  between  the  poles  of  an  annular  field-mag- 
net D,  formed  with  polar  projections  wound  with  coils  E,  that 
magnetize  the  said  projections.  The  coils  E  are  included  in  the 
circuit  of  a  generator  F,  by  means  of  which  the  field-magnet  is 
energized.  If  thus  constucted,  the  machine  is  a  well-known 
form  of  alternating-current  generator.  To  adapt  it  to  his  sys- 
tem, however,  Mr.  Tesla  winds  on  armature  c  a  second  set  of 
coils  B  B  intermediate  to  the  first,  or,  in  other  words,  in  such  po- 
sitions that  while  the  coils  of  one  set  are  in  the  relative  positions 
to  the  poles  of  the  field-magnet  to  produce  the  maximum  current, 
those  of  the  other  set  will  be  in  the  position  in  which  they  pro- 
duce the  minimum  current.  The  coils  B  are  connected,  also,  in 


FIG.  30. 


FIG.  81. 


series  and  to  two  connecting-rings,  secured  generally  to  the 
shaft  at  the  opposite  end  of  the  armature. 

The  motor  shown  in  Fig.  29  has  an  annular  field-magnet  H, 
with  four  pole-pieces  wound  with  coils  i.  The  armature  is  con- 
structed similarly  to  the  generator,  but  with  two  sets  of  two 
coils  in  closed  circuits  to  correspond  with  the  reduced  number  of 
magnetic  poles  in  the  field.  From  the  foregoing  it  is  evident  that 
one  revolution  of  the  armature  of  the  generator  producing  eight 
current  impulses  in  each  circuit  will  produce  two  revolutions  of 
the  motor-armature. 

The  application  of  the  principle  of  this  invention  is  not,  how- 
ever, confined  to  any  particular  form  of  machine.  In  Figs.  30 
and  31  a  generator  and  motor  of  another  well-known  type  are 
shown.  In  Fig.  30,  j  j  are  magnets  disposed  in  a  circle  and 
wound  with  coils  K,  which  are  in  circuit  with  a  generator  which 


POLYPHASE  CURRENTS.  39 

supplies  the  current  that  maintains  the  field  of  force.  In  the 
usual  construction  of  these  machines  the  armature-conductor  L  is 
carried  by  a  suitable  frame,  so  as  to  be  rotated  in  face  of  the 
magnets  j  .1,  or  between  these  magnets  and  another  similar  set 
in  front  of  them.  The  magnets  are  energized  so  as  to  be  of  al- 
ternately opposite  polarity  throughout  the  series,  so  that  as  the 
conductor  c  is  rotated  the  current  impulses  combine  or  are 
added  to  one  another,  those  produced  by  the  conductor  in  any 
given  position  being  all  in  the  same  direction.  To  adapt  such 
a  machine  to  his  system,  Mr.  Tesla  adds  a  second  set  of  induced 
conductors  M,  in  all  respects  similar  to  the  first,  but  so  placed 
in  reference  to  it  that  the  currents  produced  in  each  will  differ 
by  a  quarter-phase.  With  such  relations  it  is  evident  that  as  the 
current  decreases  in  conductor  L  it  increases  in  conductor  M,  and 
conversely,  and  that  any  of  the  forms  of  Tesla  motor  invented 
for  use  in  this  system  may  be  operated  by  such  a  generator. 

Fig.  31  is  intended  to  show  a  motor  corresponding  to  the  ma- 
chine in  Fig.  30.  The  construction  of  the  motor  is  identical  with 
that  of  the  generator,  and  if  coupled  thereto  it  will  run  syn- 
chronously therewith,  j'  j'  are  the  field-magnets,  and  K'  the 
coils  thereon,  i/  is  one  of  the  armature-conductors  and  M'  the 
other. 

Fig.  32  shows  in  diagram  other  forms  of  machine.  The  gene- 
rator N  in  this  case  is  shown  as  consisting  of  a  stationary  ring  o, 
wound  with  twenty-four  coils  p  p',  alternate  coils  being  connected 
in  series  in  two  circuits.  Within  this  ring  is  a  disc  or  drum  Q, 
with  projections  Q'  wound  with  energizing-coils  included  in  cir- 
cuit with  a  generator  K.  By  driving  this  disc  or  cylinder  alter- 
nating currents  are  produced  in  the  coils  p  and  p',  which  are 
carried  off  to  run  the  several  motors. 

The  motors  are  composed  of  a  ring  or  annular  field-magnet  s, 
wound  with  two  sets  of  energizing-coils  T  T',  and  armatures  u, 
having  projections  LT/  wound  with  coils  v,  all  connected  in  series 
in  a  closed  circuit  or  each  closed  independently  on  itself. 

Suppose  the  twelve  generator-coils  p  are  wound  alternately  in 
opposite  directions,  so  that  any  two  adjacent  coils  of  the  same  set 
tend  to  produce  a  free  pole  in  the  ring  o  between  them  and  the 
twelve  coils  p'  to  be  similarly  wound.  A  single  revolution  of 
the  disc  or  cylinder  Q,  the  twelve  polar  projections  of  which  are 
of  opposite  polarity,  will  therefore  produce  twelve  current  im- 
pulses in  each  of  the  circuits  w  w'.  Hence  the  motor  x,  which 


40  INVENTIONS  OF  NIKOLA  TE8LA. 

has  sixteen  coils  or  eight  free  poles,  will  make  one  and  a  half  turns 
to  the  generator's  one.  The  motor  Y,  with  twelve  coils  or  six 
poles,  will  rotate  with  twice  the  speed  of  the  generator,  and  the 
motor  z,  with  eight  coils  or  four  poles,  will  revolve  three  times 
as  fast  as  the  generator.  These  multipolar  motors  have  a  peculi- 
arity which  may  be  often  utilized  to  great  advantage.  For  ex- 


FTG.  32. 

ample,  in  the  motor  x,  Fig.  32,  the  eight  poles  may  be  either 
alternately  opposite  or  there  may  be  at  any  given  time  alternately 
two  like  and  two  opposite  poles.  This  is  readily  attained  by 
making  the  proper  electrical  connections.  The  effect  of  such  a 
change,  however,  would  be  the  same  as  reducing  the  number  of 


POLYPHASE  CURRENTS.  41 

poles  one-half,  and  thereby  doubling  the  speed  of  any  given 
motor. 

It  is  obvious  that  the  Tesla  electrical  transformers  which  have 
independent  primary  currents  may  be  used  with  the  generators 
described.  It  may  also  be  stated  with  respect  to  the  devices 
we  now  describe  that  the  most  perfect  and  harmonious  action 
of  the  generators  and  motors  is  obtained  when  the  numbers  of  the 
poles  of  each  are  even  and  not  odd.  If  this  is  not  the  case,  there 
will  be  a  certain  unevenness  of  action  which  is  the  less  appreci- 
able as  the  number  of  poles  is  greater;  although  this  may  be  in  a 
measure  corrected  by  special  provisions  which  it  is  not  here 
necessary  to  explain.  It  also  follows,  as  a  matter  of  course,  that 
if  the  number  of  the  poles  of  the  motor  be  greater  than  that  of 
the  generator  the  motor  will  revolve  at  a  slower  speed  than  the 
generator. 

In  this  chapter,  we  may  include  a  method  devised  by  Mr. 
Tesla  for  avoiding  the  very  high  speeds  which  would  be  neces- 
sary with  large  generators.  In  lieu  of  revolving  the  generator 
armature  at  a  high  rate  of  speed,  he  secures  the  desired  result  by 
a  rotation  of  the  magnetic  poles  of  one  element  of  the  generator, 
while  driving  the  other  at  a  different  speed.  The  effect  is  the 
same  as  that  yielded  by  a  very  high  rate  of  rotation. 

In  this  instance,  the  generator  which  supplies  the  current  for 
operating  the  motors  or  transformers  consists  of  a  subdivided 
ring  or  annular  core  wound  with  four  diametrically-opposite 
coils,  E  F/,  Fig.  33.  Within  the  ring  is  mounted  a  cylindrical 
armature-core  wound  longitudinally  with  two  independent  coils, 
F  F',  the  ends  of  which  lead,  respectively,  to  two  pairs  of  insu- 
lated contact  or  collecting  rings,  D  D'  G  G',  on  the  armature  shaft. 
Collecting  brushes  d  d'  g  g'  bear  upon  these  rings,  respectively, 
and  convey  the  currents  through  the  two  independent  line-cir- 
cuits M  M'.  In  the  main  line  there  may  be  included  one  or  more 
motors  or  transformers,  or  both.  If  motors  be  used,  they  are  of 
the  usual  form  of  Tesla  construction  with  independent  coils  or 
sets  of  coils  j  j',  included,  respectively,  in  the  circuits  M  M'. 
These  energizing-coils  are  wound  on  a  ring  or  annular  field  or  on 
pole  pieces  thereon,  and  produce  by  the  action  of  the  alternating 
currents  passing  through  them  a  progressive  shifting  of  the  mag- 
netism from  pole  to  pole.  The  cylindrical  armature  H  of  the 
motor  is  wound  with  two  coils  at  right  angles,  which  form  inde- 
pendent closed  circuits. 


42  INVENTIONS  OF  NIKOLA  TESLA. 

If  transformers  be  employed,  one  set  of  the  primary  coils,  as 
N  N,  wound  on  a  ring  or  annular  core  is  connected  to  one  circuit, 
as  M',  and  the  other  primary  coils,  N  N',  to  the  circuit  M.  The 
secondary  coils  K  K'  may  then  be  utilized  for  running  groups  of 
incandescent  lamps  p  p'. 

With  this  generator  an  exciter  is  employed.     This  consists  of 


FIG.  33. 

two  poles,  A  A,  of  steel  permanently  magnetized,  or  of  iron  ex- 
cited by  a  battery  or  other  generator  of  continuous  currents,  and 
a  cylindrical  armature  core  mounted  on  a  shaft,  B,  and  wound 
with  two  longitudinal  coils,  c  c'.  One  end  of  each  of  these  coils 
is  connected  to  the  collecting-rings  I  c,  respectively,  while  the 


POLYPHASE  CURRENTS.  43 

other  ends  are  both  connected  to  a  ring,  a.  Collecting-brushes 
b'  e'  bear  on  the  rings  b  c,  respectively,  and  conductors  L  L  con- 
vey tlie  currents  therefrom  through  the  coils  E  and  E  of  the  gen- 
erator, i/  is  a  common  return-wire  to  brush  a'.  Two  indepen- 
dent circuits  are  thus  formed,  one  including  coils  c  of  the  exciter 
and  E  E  of  the  generator,  the  other  coils  c'  of  the  exciter  and  E' 
E'  of  the  generator.  It  results  from  this  that  the  operation  of 
the  exciter  produces  a  progressive  movement  of  the  magnetic 
poles  of  the  annular  field-core  of  the  generator,  the  shifting  or 
rotary  movement  of  the  poles  being  synchronous  with  the  rota- 
tion of  the  exciter  armature.  Considering  the  operative  con- 
ditions of  a  system  thus  established,  it  will  be  found  that  when 
the  exciter  is  driven  so  as  to  energize  the  field  of  the  generator, 
the  armature  of  the  latter,  if  left  free  to  turn,  would  rotate  at  a 
speed  practically  the  same  as  that  of  the  exciter.  If  under  such 
conditions  the  coils  F  F'  of  the  generator  armature  be  closed 
upon  themselves  or  short-circuited,  no  currents,  at  least  theoreti- 
cally, will  be  generated  in  these  armature  coils.  In  practice 
the  presence  of  slight  currents  is  observed,  the  existence  of  which 
is  attributable  to  more  or  less  pronounced  fluctuations  in  the  in- 
tensity of  the  magnetic  poles  of  the  generator  ring.  So,  if  the 
armature-coils  F  F'  be  closed  through  the  motor,  the  latter  will 
not  be  turned  as  long  as  the  movement  of  the  generator  armature 
is  synchronous  with  that  of  the  exciter  or  of  the  magnetic  poles 
of  its  lield.  If,  on  the  contrary,  the  speed  of  the  generator  arm- 
ature be  in  any  way  checked,  so  that  the  shifting  or  rotation  of 
the  poles  of  the  field  becomes  relatively  more  rapid,  currents  will 
be  induced  in  the  armature  coils.  This  obviously  follows  from 
the  passing  of  the  lines  of  force  across  the  armature  conductors. 
The  greater  the  speed  of  rotation  of  the  magnetic  poles  relatively 
to  that  of  the  armature  the  more  rapidly  the  currents  developed 
in  the  coils  of  the  latter  will  follow  one  another,  and  the  more 
rapidly  the  motor  will  revolve  in  response  thereto,  and  this  con- 
tinues until  the  armature  generator  is  stopped  entirely,  as  by  a 
brake,  when  the  motor,  if  properly  constructed,  runs  at  the  speed 
with  which  the  magnetic  poles  of  the  generator  rotate. 

The  effective  strength  of  the  currents  developed  in  the  arma- 
ture coils  of  the  generator  is  dependent  upon  the  strength  of  the 
currents  energizing  the  generator  and  upon  the  number  of  rota- 
tions per  unit  of  time  of  the  magnetic  poles  of  the  generator; 
hence  the  speed  of  the  motor  armature  will  depend  in  all  cases 


44  INVENTIONS  OF  NIKOLA  TESLA. 

upon  the  relative  speeds  of  the  armature  of  the  generator  and  of 
its  magnetic  poles.  For  example,  if  the  poles  are  turned  two 
thousand  times  per  unit  of  time  and  the  armature  is  turned  eight 
hundred,  the  motor  will  turn  twelve  hundred  times,  or  nearly  so. 
Very  slight  diiferences  of  speed  may  be  indicated  by  a  delicately 
balanced  motor. 

Let  it  now  be  assumed  that  power  is  applied  to  the  generator 
armature  to  turn  it  in  a  direction  opposite  to  that  in  which  its 
magnetic  poles  rotate.  In  such  case  the  result  would  be  similar 
to  that  produced  by  a  generator  the  armature  and  field  magnets 
of  which  are  rotated  in  opposite  directions,  and  by  reason  of  these 
conditions  the  motor  armature  will  turn  at  a  rate  of  speed  equal 
to  the  sum  of  the  speeds  of  the  armature  and  magnetic  poles  of 
the  generator,  so  that  a  comparatively  low  speed  of  the  generator 
armature  will  produce  a  high  speed  in  the  motor. 

It  will  be  observed  in  connection  with  this  system  that  on 
diminishing  the  resistance  of  the  external  circuit  of  the  generator 
armature  by  checking  the  speed  of  the  motor  or  by  adding 
translating  devices  in  multiple  arc  in  the  secondary  circuit  or  cir- 
cuits of  the  transformer  the  strength  of  the  current  in  the  arma- 
ture circuit  is  greatly  increased.  This  is  due  to  two  causes :  first, 
to  the  great  differences  in  the  speeds  of  the  motor  and  generator, 
and,  secondly,  to  the  fact  that  the  apparatus  follows  the  analogy 
of  a  transformer,  for,  in  proportion  as  the  resistance  of  the  arma- 
ture or  secondary  circuits  is  reduced,  the  strength  of  the  currents 
in  the  field  or  primary  circuits  of  the  generator  is  increased  and 
the  currents  in  the  armature  are  augmented  correspondingly. 
For  similar  reasons  the  currents  in  the  armature-coils  of  the 
generator  increase  very  rapidly  when  the  speed  of  the  armature 
is  reduced  when  running  in  the  same  direction  as  the  magnetic 
poles  or  conversely. 

It  will  be  understood  from  the  above  description  that  the 
generator-armature  may  be  run  in  the  direction  of  the  shifting  of 
the  magnetic  poles,  but  more  rapidly,  and  that  in  such  case  the 
speed  of  the  motor  will  be  equal  to  the  difference  between  the 
two  rates. 


CHAPTER  VII. 

FOR    RoTARY    CURRENT    MoTORS. 

AN  interesting  device  for  regulating  and  reversing  has  been 
devised  by  Mr.  Tesla  for  the  purpose  of  varying  the  speed  of 
polyphase  motors.  It  consists  of  a  form  of  converter  or  trans- 
former with  one  element  capable  of  movement  with  respect  to 
the  other,  whereby  the  inductive  relations  may  be  altered,  either 
manually  or  automatically,  for  the  purpose  of  varying  the 
strength  of  the  induced  current.  Mr.  Tesla  prefers  to  construct 
this  device  in  such  manner  that  the  induced  or  secondary  ele- 
ment may  be  movable  with  respect  to  the  other ;  and  the  inven- 
tion, so  far  as  relates  merely  to  the  construction  of  the  device  it- 
self, consists,  essentially,  in  the  combination,  with  two  opposite 
magnetic  poles,  of  an  armature  wound  with  an  insulated  coil  and 
mounted  on  a  shaft,  whereby  it  may  be  turned  to  the  desired 
extent  within  the  field  produced  by  the  poles.  The  normal  po- 
sition of  the  core  of  the  secondary  element  is  that  in  which  it 
most  completely  closes  the  magnetic  circuit  between  the  poles 
of  the  primary  element,  and  in  this  position  its  coil  is  in  its 
most  effective  position  for  the  inductive  action  upon  it  of  the 
primary  coils ;  but  by  turning  the  movable  core  to  either  side, 
the  induced  currents  delivered  by  its  coil  become  weaker  until, 
by  a  movement  of  the  said  core  and  coil  through  90°,  there  will 
be  no  current  delivered. 

Fig.  34  is  a  view  in  side  elevation  of  the  regulator.  Fig.  35  is 
a  broken  section  on  line  a1  a?  of  Fig.  34.  Fig.  36  is  a  diagram 
illustrating  the  most  convenient  manner  of  applying  the  regulator 
to  ordinary  forms  of  motors,  and  Fig.  37  is  a  similar  diagram  illus- 
trating the  application  of  the  device  to  the  Tesla  alternating- 
current  motors.  The  regulator  may  be  constructed  in  many 
ways  to  secure  the  desired  result ;  but  that  which  is,  perhaps,  its 
best  form  is  shown  in  Figs.  34  and  35. 

A  represents  a  frame  of  iron.     B  B  are  the  cores  of  the  indue- 


Iti 


INVENTIONS  OF  NIKOLA  TEKLA. 


ing  or  primary  coils  c  c.  i>  is  a  shaft  mounted  on  the  side  bars, 
D',  and  on  which  is  secured  a  sectional  iron  core,  E,  wound  with 
an'  induced  or  secondary  coil,  F,  the  convolutions  of  which  are 
parallel  with  the  axis  of  the  shaft.  The  ends  of  the  core  are 
rounded  off  so  as  to  fit  closely  in  the  space  between  the  two  poles 
and  permit  the  core  E  to  be  turned  to  and  held  at  any  desiivd 
point.  A  handle,  G,  secured  to  the  projecting  end  of  the  shaft 
D,  is  provided  for  this  purpose. 

In  Fig.  36  let  n  represent  an  ordinary  alternating  current  gen- 
erator, the  field-magnets  of  which  are  excited  by  a  suitable 
source  of  current,  i.  Let  j  designate  an  ordinary  form  of  electro- 
magnetic motor  provided  with  an  armature,  K,  commutator  L, 
and  field-magnets  M.  It  is  well  known  that  such  a  motor,  if  its 


FIG.  34. 


field-magnet  cores  be  divided  up  into  insulated  sections,  may  be 
practically  operated  by  an  alternating  current ;  but  in  using  this 
regulator  with  such  a  motor,  Mr.  Tesla  includes  one  element  of 
the  motor  only — say  the  armature-coils — in  the  main  circuit  of 
the  generator,  making  the  connections  through  the  brushes  and 
the  commutator  in  the  usual  way.  He  also  includes  one  of  the 
elements  of  the  regulator — say  the  stationary  coils — in  the  same 
circuit,  and  in  the  circuit  with  the  secondary  or  movable  coil  of 
the  regulator  he  connects  up  the  field-coils  of  the  motor.  He 
also  prefers  to  use  flexible  conductors  to  make  the  connections 
from  the  secondary  coil  of  the  regulator,  as  he  thereby  avoids 
the  use  of  sliding  contacts  or  rings  without  interfering  with  the 
requisite  movement  of  the  core  E. 


POLYPHASE  CURRENTS. 


47 


If  the  regulator  be  in  its  normal  position,  or  that  in  which  its 
magnetic  circuit  is  most  nearly  closed,  it  delivers  its  maximum 
induced  current,  the  phases  of  which  so  correspond  with  those  of 
the  primary  current  that  the  motor  will  run  as  though  both  lield 
and  armature  were  excited  by  the  main  current. 

To  vary  the  speed  of  the  motor  to  any  rate  between  the  mini- 
mum and  maximum  rates,  the  core  E  and  coils  F  are  turned  in 
either  direction  to  an  extent  which  produces  the  desired  result, 
for  in  its  normal  position  the  convolutions  of  coil  F  embrace  the 
maximum  number  of  lines  of  force,  all  of  which  act  with  the 
same  effect  upon  the  coil ;  hence  it  will  deliver  its  maximum 
current ;  but  by  turning  the  coil  F  out  of  its  position  of  maximum 
effect  the  number  of  lines  of  force  embraced  by  it  is  diminished. 
The  inductive  effect  is  therefore  impaired,  and  the  current  de- 
livered by  coil  F  will  continue  to  diminish  in  proportion  to  the 
angle  at  which  the  coil  F  is  turned  until,  after  passing  through 


FIG.  36. 

an  angle  of  ninety  degrees,  the  convolutions  of  the  coil  will  be 
at  right  angles  to  those  of  coils  c  c,  and  the  inductive  effect  re- 
duced to  a  minimum. 

Incidentally  to  certain  constructions,  other  causes  may  influ- 
ence the  variation  in  the  strength  of  the  induced  currents.  For 
example,  in  the  present  case  it  will  be  observed  that  by  the  first 
movement  of  coil  F  a  certain  portion  of  its  convolutions  are  carried 
beyond  the  line  of  the  direct  influence  of  the  lines  of  force,  and 
that  the  magnetic  path  or  circuit  for  the  lines  is  impaired ;  hence 
the  inductive  effect  would  be  reduced.  Next,  that  after  moving 
through  a  certain  angle,  which  is  obviously  determined  by  the 
relative  dimensions  of  the  bobbin  or  coil  F,  diagonally  opposite 
portions  of  the  coil  will  be  simultaneously  included  in  the  field, 
but  in  such  positions  that  the  lines  which  produce  a  current- 
impulse  in  one  portion  of  the  coil  in  a  certain  direction  will  pro- 


INVENTIONS  OF  NIKOLA  TESLA. 


duce  in  the  diagonally  opposite  portion  a  corresponding  impulse 
in  the  opposite  direction;  hence  portions  of  the  current  will 
neutralize  one  another. 

As  before  stated,  the  mechanical  construction  of  the  device 
may  be  greatly  varied ;  but  the  essential  conditions  of  the  princi- 
ple will  be  fulfilled  in  any  apparatus  in  which  the  movement  of 
the  elements  with  respect  to  one  another  effects  the  same  results 
by  varying  the  inductive  relations  of  the  two  elements  in  a  man- 
ner similar  to  that  described. 

It  may  also  be  stated  that  the  core  E  is  not  indispensable  to  the 
operation  of  the  regulator ;  but  its  presence  is  obviously  bene- 
ficial. This  regulator,  however,  has  another  valuable  property 
in  its  capability  of  reversing  the  motor,  for  if  the  coil  F  be  turned 


through  a  half-revolution,  the  position  of  its  convolutions  rela- 
tively to  the  two  coils  c  c  and  to  the  lines  of  force  is  reversed,  and 
consequently  the  phases  of  the  current  will  be  reversed.  This 
will  produce  a  rotation  of  the  motor  in  an  opposite  direction. 
This  form  of  regulator  is  also  applied  with  great  advantage  to 
Mr.  Tesla's  system  of  utilizing  alternating  currents,  in  which  the 
magnetic  poles  of  the  field  of  a  motor  are  progressively  shifted 
by  means  of  the  combined  effects  upon  the  field  of  magnetizing 
coils  included  in  independent  circuits,  through  which  pass  alter- 
nating currents  in  proper  order  and  relations  to  each  other. 

In  Fig.  37,  let  P  represent  a  Tesla  generator  having  two  inde- 
pendent coils,  P'  and  P",  on  the  armature,  and  T  a  diagram  of  a 


POL7PHAHE  CURRENTS.  49 

motor  having  two  independent  energizing  coils  or  sets  of  coils, 
R  R'.  One  of  the  circuits  from  the  generator,  as  s'  s',  includes 
one  set,  R'  R',  of  the  energizing  coils  of  the  motor,  while  the 
other  circuit,  as  s  s,  includes  the  primary  coils  of  the  regulator. 
The  secondary  coil  of  the  regulator  includes  the  other  coils,  R  R, 
of  the  motor. 

While  the  secondary  coil  of  the  regulator  is  in  its  normal  posi- 
tion, it  produces  its  maximum  current,  and  the  maximum  rotary 
effect  is  imparted  to  the  motor;  but  this  effect  will  be  diminished 
in  proportion  to  the  angle  at  which  the  coil  F  of  the  regulator  is 
turned.  The  motor  will  also  be  reversed  by  reversing  the  posi- 
tion of  the  coil  with  reference  to  the  coils  c  c,  and  thereby  re- 
versing the  phases  of  the  current  produced  by  the  generator.  This 
changes  the  direction  of  the  movement  of  the  shifting  poles  which 
the  armature  follows. 

One  of  the  main  advantages  of  this  plan  of  regulation  is  its 
economy  of  power.  When  the  induced  coil  is  generating  its 
maximum  current,  the  maximum  amount  of  energy  in  the  prim- 
ary coils  is  absorbed  ;  but  as  the  induced  coil  is  turned  from  its 
normal  position  the  self-induction  of  the  primary-coils  reduces 
the  expenditure  of  energy  and  saves  power. 

It  is  obvious  that  in  practice  either  coils  c  <:  or  coil  v  may  be 
used  as  primary  or  secondary,  and  it  is  well  understood  that  their 
relative  proportions  may  be  varied  to  produce  any  desired  differ- 
ence or  similarity  in  the  inducing  and  induced  currents. 


CHAPTER  VIII. 

SINGLE  CIRCUIT,  SELF-STARTING  SYNCHRONIZING  MOTORS. 

In  the  first  chapters  of  this  section  we  have,  bearing  in  mind 
the  broad  underlying  principle,  considered  a  distinct  class  of  mo- 
tors, namely,  such  as  require  for  their  operation  a  special  genera- 
tor capable  of  yielding  currents  of  differing  phase.  As  a  matter 
of  course,  Mr.  Tesla  recognizing  the  desirability  of  utilizing  his 
motors  in  connection  with  ordinary  systems  of  distribution,  ad- 
dressed himself  to  the  task  of  inventing  various  methods  and 
ways  of  achieving  this  object.  In  the  succeeding  chapters, 
therefore,  we  witness  the  evolution  of  a  number  of  ideas  bearing 
upon  this  important  branch  of  work.  It  must  be  obvious  to 
a  careful  reader,  from  a  number  of  hints  encountered  here  and 
there,  that  even  the  inventions  described  in  these  chapters  to  fol- 
low do  not  represent  the  full  scope  of  the  work  done  in  these 
lines.  They  might,  indeed,  be  regarded  as  exemplifications. 

We  will  present  these  various  inventions  in  the  order  which 
to  us  appears  the  most  helpful  to  an  understanding  of  the  subject 
by  the  majority  of  readers.  It  will  be  naturally  perceived  that 
in  offering  a  series  of  ideas  of  this  nature,  wherein  some  of  the 
steps  or  links  are  missing,  the  descriptions  are  not  altogether  se- 
quential; but  any  one  who  follows  carefully  the  main  drift  of 
the  thoughts  now  brought  together  will  find  that  a  satisfactory 
comprehension  of  the  principles  can  be  gained. 

As  is  well  known,  certain  forms  of  alternating-current  machines 
have  the  property,  when  connected  in  circuit  with  an  alternating 
current  generator,  of  running  as  a  motor  in  synchronism  there- 
with ;  but,  while  the  alternating  current  will  run  the  motor  after 
it  has  attained  a  rate  of  speed  synchronous  with  that  of  the  gen- 
erator, it  will  not  start  it.  Hence,  in  all  instances  heretofore 
where  these  "  synchronizing  motors,"  as  they  are  termed,  have 
been  run,  some  means  have  been  adopted  to  bring  the  motors  up 
to  synchronism  with  the  generator,  or  approximately  so,  before 
the  alternating  current  of  the  generator  is  applied  to  drive  them. 


POLYPHASE  CURRENTS.  51 

In  some  instances  mechanical  appliances  have  been  utilized  for 
this  purpose.  In  others  special  and  complicated  forms  of  motor 
have  been  constructed.  Mr.  Tesla  has  discovered  a  much  more 
simple  method  or  plan  of  operating  synchronizing  motors,  which 
requires  practically  no  other  apparatus  than  the  motor  itself.  In 
other  words,  by  a  certain  change  in  the  circuit  connections  of  the 
motor  he  converts  it  at  will  from  a  double  circuit  motor,  or  such 
as  have  been  already  described,  and  which  will  start  under  the 
action  of  an  alternating  current,  into  a  synchronizing  motor,  or 
one  which  will  be  run  by  the  generator  only  when  it  has  reached 
a  certain  speed  of  rotation  synchronous  with  that  of  the  genera- 
tor. In  this  manner  he  is  enabled  to  extend  very  greatly  the  ap- 
plications of  his  system  and  to  secure  all  the  advantages  of  both 
forms  of  alternating  current  motor. 

The  expression  "  synchronous  with  that  of  the  generator,"  is 
used  here  in  its  ordinary  acceptation — that  is  to  say,  a  motor  is 
said  to  synchronize  with  the  generator  when  it  preserves  a  certain 
relative  speed  determined  by  its  number  of  poles  and  the  number 
of  alternations  produced  per  revolution  of  the  generator.  Its 
actual  speed,  therefore,  may  be  faster  or  slower  than  that  of  the 
generator;  but  it  is  said  to  be  synchronous  so  long  as  it  preserves 
the  same  relative  speed. 

In  carrying  out  this  invention  Mr.  Tesla  constructs  a  motor 
which  has  a  strong  tendency  to  synchronism  with  the  generator. 
The  construction  preferred  is  that  in  which  the  armature  is  pro- 
vided with  polar  projections.  The  field-magnets  are  wound  with 
two  sets  of  coils,  the  terminals  of  which  are  connected  to  a  switch 
mechanism,  by  means  of  which  the  line-current  may  be  carried 
directly  through  these  coils  or  indirectly  through  paths  by 
which  its  phases  are  modified.  To  start  such  a  motor,  the  switch 
is  turned  on  to  a  set  of  contacts  which  includes  in  one  motor 
circuit  a  dead  resistance,  in  the  other  an  inductive  resistance,  and, 
the  two  circuits  being  in  derivation,  it  is  obvious  that  the  differ- 
ence in  phase  of  the  current  in  such  circuits  will  set  up  a  rotation 
of  the  motor.  When  the  speed  of.  the  motor  has  thus  been 
brought  to  the  desired  rate  the  switch  is  shifted  to  throw  the 
main  current  directly  through  the  motor-circuits,  and  although 
the  currents  in  both  circuits  will  now  be  of  the  same  phase  the 
motor  will  continue  to  revolve,  becoming  a  true  synchronous 
motor.  To  secure  greater  efficiency,  the  armature  or  its  polar 
projections  are  wound  with  coils  closed  on  themselves. 


53  INVENTIONS  OF  NIKOLA  TESLA. 

In  the  accompanying  diagrams,  Fig.  38  illustrates  the  details 
of  the  plan  above  set  forth,  and  Figs.  39  and  40  modifications 
of  the  same. 

Referring  to  Fig.  38,  let  A  designate  the  neld-magnets  of  a 


FK;S.  : 


motor,  the  polar  projections  of  which  are  wound  with  coils  is  c 
included  in  independent  circuits,  and  D  the  armature  with  polar 
projections  wound  with  coils  E  closed  upon  themselves,  the 
motor  in  these  respects  being  similar  in  construction  to  those 


POLYPHASE  CURRENTS.  53 

described  already,  but  having  OH  account  of  the  polar  projections 
on  the  armature  core,  or  other  similar  and  well-known  features, 
the  properties  of  a  synch ronizing-motor.  L  i/  represents  the 
conductors  of  a  line  from  an  alternating  current  generator  <j. 

Near  the  motor  is  placed  a  switch  the  action  of  which  is  that 
of  the  one  shown  in  the  diagrams,  which  is  constructed  as  fol- 
lows :  F  F'  are  two  conducting  plates  or  arms,  pivoted  at  their 
ends  and  connected  by  an  insulating  cross-bar,  H,  so  as  to  be 
shifted  in  parallelism.  In  the  path  of  the  bars  F  F7  is  the  contact 
•2,  which  forms  one  terminal  of  the  circuit  through  coils  c,  and 
the  contact  4,  which  is  one  terminal  of  the  circuit  through  coils 
B.  The  opposite  end  of  the  wire  of  coils  c  is  connected  to  the 
wire  L  or  bar  F'  ,  and  the  corresponding  end  of  coils  B  is  connected 
to  wire  i/  and  bar  F;  hence  if  the  bars  be  shifted  so  as  to  bear  on 
contacts  2  and  4  both  sets  of  coils  B  c:  will  be  included  in  the  cir- 
cuit L  i/  in  multiple  arc  or  derivation.  In  the  path  of  the  levers 
F  F'  are  two  other  contact  terminals,  L  and  3.  The  contact  1  is 
connected  to  contact  2  through  an  artificial  resistance,  i,  and  con- 
tact 3  with  contact  4  through  a  self-induction  coil,  j,  so  that  when 
the  switch  levers  are  shifted  upon  the  points  ]  and  3  the  circuits 
of  coils  B  and  c  will  be  connected  in  multiple  arc  or  derivation  to 
the  circuit  L  i/,  and  will  include  the  resistance  and  self-induction 
coil  respectively.  A  third  position  of  the  switch  is  that  in  which 
the  levers  F  and  F'  are  shifted  out  of  contact  with  both  sets  of 
points.  In  this  case  the  motor  is  entirely  out  of  circuit. 

The  purpose  and  manner  of  operating  the  motor  by  these  de- 
vices are  as  follows :  The  normal  position  of  the  switch,  the 
motor  being  out  of  circuit,  is  off  the  contact  points.  Assuming 
the  generator  to  be  running,  and  that  it  is  desired  to  start  the 
motor,  the  switch  is  shifted  until  its  levers  rest  upon  points  1  and 
3.  The  two  motor-circuits  are  thus  connected  with  the  generator 
circuit ;  but  by  reason  of  the  presence  of  the  resistance  i  in  one 
and  the  self-induction  coil  j  in  the  other  the  coincidence  of  the 
phases  of  the  current  is  disturbed  sufficiently  to  produce  a  pro- 
gression of  the  poles,  which  starts  the  motor  in  rotation.  When 
tl.'e  speed  of  the  motor  has  run  up  to  synchronism  with  the 
generator,  or  approximately  so,  the  switch  is  shifted  over  upon 
the  points  2  and  4,  thus  cutting  out  the  coils  i  and  j,  so  that  the 
currents  in  both  circuits  have  the  same  phase;  but  the  motor 
now  runs  as  a  synchronous  motor. 

It  will  be  understood  that  when  brought  up  to  speed  the  mo 


r>4  INVENTIONS  OF  NIKOLA  TESLA. 

tor  will  run  with  only  one  of  the  circuits  B  or  c  connected  with 
the  main  or  generator  circuit,  or  the  two  circuits  may  be  con- 
nected in  series.  This  latter  plan  is  preferable  when  a  current 
having  a  high  number  of  alternations  per  unit  of  time  is  em- 
ployed to  drive  the  motor.  In  such  case  the  starting  of  the 
motor  is  more  difficult,  and  the  dead  and  inductive  resistances 
must  take  up  a  considerable  proportion  of  the  electromotive 
force  of  the  circuits.  Generally  the  conditions  are  so  adjusted 
that  the  electromotive  force  used  in  each  of  the  motor  circuits  is 
that  which  is  required  to  operate  the  motor  when  its  circuits  are 
in  series.  The  plan  followed  in  this  case  is  illustrated  in  Fig. 
39.  In  this  instance  the  motor  has  twelve  poles  and  the  arma- 
ture has  polar  projections  D  wound  with  closed  coils  E.  The 
switch  used  is  of  substantially  the  same  construction  as  that 
shown  in  the  previous  figure.  There  are,  however,  five  contacts, 
designated  as  5,  6,  7,  8,  and  9.  The  motor-circuits  B  c,  which  in- 
clude alternate  field-coils,  are  connected  to  the  terminals  in  the 
following  order :  One  end  of  circuit  c  is  connected  to  contact  9 
and  to  contact  5  through  a  dead  resistance,  i.  One  terminal  of 
circuit  B  is  connected  to  contact  7  and  to  contact  6  through  a 
self-induction  coil,  J.  The  opposite  terminals  of  both  circuits  are 
connected  to  contact  8. 

One  of  the  levers,  as  F,  of  the  switch  is  made  with  an  exten- 
sion, /,  or  otherwise,  so  as  to  cover  both  contacts  5  and  6  when 
shifted  into  the  position  to  start  the  motor.  It  will  be  observed 
that  when  in  this  position  and  with  lever  F'  on  contact  8  the  cur- 
rent divides  between  the  two  circuits  B  c,  which  from  their  dif- 
ference in  electrical  character  produce  a  progression  of  the  poles 
that  starts  the  motor  in  rotation.  When  the  motor  has  attained 
the  proper  speed,  the  switch  is  shifted  so  that  the  levers  cover 
the  contacts  7  and  9,  thereby  connecting  circuits  B  and  c  in  se- 
ries. It  is  found  that  by  this  disposition  the  motor  is  maintained 
in  rotation  in  synchronism  with  the  generator.  This  principle 
of  operation,  which  consists  in  converting  by  a  change  of  con- 
nections or  otherwise  a  double-circuit  motor,  or  one  operating  by 
a  progressive  shifting  of  the  poles,  into  an  ordinary  synchroniz- 
ing motor  may  be  carried  out  in  many  other  ways.  For  instance, 
instead  of  using  the  switch  shown  in  the  previous  figures,  we 
may  use  a  temporary  ground  circuit  between  the  generator  and 
motor,  in  order  to  start  the  motor,  in  substantially  the  manner 
indicated  in  Fig.  40.  Let  G  in  this  figure  represent  an  ordinary 


POLYPHASE  CURRENTS.  55 

alternating-current  generator  with,  say,  two  poles,  M  M',  and  an 
armature  wound  with  two  coils,  N  N',  at  right  angles  and  con- 
nected in  series.  The  motor  has,  for  example,  four  poles  wound 
with  coils  B  c,  which  are  connected  in  series,  and  an  armature 
with  polar  projections  D  wound  with  closed  coils  E  E.  From  the 
common  joint  or  union  between  the  two  circuits  of  both  the  gen- 
erator and  the  motor  an  earth  connection  is  established,  while 
the  terminals  or  ends  of  these  circuits  are  connected  to  the 
line.  Assuming  that  the  motor  is  a  synchronizing  motor  or  one 
that  has  the  capability  of  running  in  synchronism  with  the  gen- 
erator, but  not  of  starting,  it  may  be  started  by  the  above- 
described  apparatus  by  closing  the  ground  connection  from  both 
generator  and  motor.  The  system  thus  becomes  one  with  a  two- 
circuit  generator  and  motor,  the  ground  forming  a  common  re- 
turn for  the  currents  in  the  two  circuits  L  and  i/.  When  by 
this  arrangement  of  circuits  the  motor  is  brought  to  speed,  the 
ground  connection  is  broken  between  the  motor  or  generator,  or 
both,  ground-switches  PP'  being  employed  for  this  purpose. 
The  motor  then  runs  as  a  synchronizing  motor. 

In  describing  the  main  features  which  constitute  this  invention 
illustrations  have  necessarily  been  omitted  of  the  appliances  used 
in  conjunction  with  the  electrical  devices  of  similar  systems — 
such,  for  instance,  as  driving-belts,  fixed  and  loose  pulleys  for  the 
motor,  and  the  like ;  but  these  are  matters  well  understood. 

Mr.  Tesla  believes  he  is  the  first  to  operate  electro-magnetic 
motors  by  alternating  currents  in  any  of  the  ways  herein  described 
— that  is  to  say,  by  producing  a  progressive  movement  or  rota- 
tion of  their  poles  or  points  of  greatest  magnetic  attraction  by 
the  alternating  currents  until  they  have  reached  a  given  speed, 
and  then  by  the  same  currents  producing  a  simple  alternation  of 
their  poles,  or,  in  other  words,  by  a  change  in  the  order  or  char- 
acter of  the  circuit  connections  to  convert  a  motor  operating  on 
one  principle  to  one  operating  on  another. 


CHAPTER  IX. 

CHANGE    FROM    DOUBLE  CURRENT  TO  SINGLE  CURRENT  MOTOR. 

A  DESCRIPTION  is  given  elsewhere  of  a  method  of  operating  al- 
ternating current  motors  by  first  rotating  their  magnetic  poles 
until  they  have  attained  synchronous  speed,  and  then  alternating 
the  poles.  The  motor  is  thus  transformed,  by  a  simple  change 
of  circuit  connections  from  one  operated  by  the  action  of  two  or 
more  independent  energizing  currents  to  one  operated  either  by 
a  single  current  or  by  several  currents  acting  as  one.  Another 
way  of  doing  this  will  now  be  described. 

At  the  start  the  magnetic  poles  of  one  element  or  field  of  the 
motor  are  progressively  shifted  by  alternating  currents  differing 
in  phase  and  passed  through  independent  energizing  circuits,  and 
short  circuit  the  coils  of  the  other  element.  When  the  motor 
thus  started  reaches  or  passes  the  limit  of  speed  synchronous  with 
the  generator,  Mr.  Tesla  connects  up  the  coils  previously  short-cir- 
cuited with  a  source  of  direct  current  and  by  a  change  of  the  cir- 
cuit connections  produces  a  simple  alternation  of  the  poles.  The 
motor  then  continues  to  run  in  synchronism  with  the  generator. 
The  motor  here  shown  in  Fig.  41  is  one  of  the  ordinary  forms,  with 
field-cores  either  laminated  or  solid  and  with  a  cylindrical  lamin- 
ated armature  wound,  for  example,  with  the  coils  A  B  at  right  angles. 
The  shaft  of  the  armature  carries  three  collecting  or  contact  rings 
c  D  E.  (Shown,  for  better  illustration,  as  of  different  diameters.) 

One  end  of  coil  A  connects  to  one  ring,  as  c,  and  one  end  of 
coil  B  connects  with  ring  D.  The  remaining  ends  are  connected 
to  ring  E.  Collecting  springs  or  brushes  F  G  H  bear  upon  the 
rings  and  lead  to  the  contacts  of  a  switch,  to  be  presently  de- 
scribed. The  field-coils  have  their  terminals  in  binding-posts  K 
K,  and  may  be  either  closed  upon  themselves  or  connected  writh 
a  source  of  direct  current  L,  by  means  of  a  switch  M.  The  main 
or  controlling  switch  has  five  contacts  a  b  c  d  e  and  two  levers/ 
g,  pivoted  and  connected  by  an  insulating  cross-bar  A,  so  as  to 
move  in  parallelism.  These  levers  are  connected  to  the  line 


POLYPHASE  CURRENTS. 


5? 


wires  from  a  source  of  alternating  currents  N.  Contact  a  is  con- 
nected to  brush  o  and  coil  B  through  a  dead  resistance  R  and 
wire  P.  Contact  b  is  connected  with  brush  F  and  coil  A  through 
a  self-induction  coil  s  and  wire  o.  Contacts  c  and  e  are  connected 
to  brushes  <;  F,  respectively,  through  the  wires  P  o,  and  contact 
<l  is  directly  connected  with  brush  H.  The  lever /has  a  widened 
end,  which  may  span  the  contacts  a  1>.  When  in  such  position 
and  with  lever  g  on  contact  d,  the  alternating  currents  divide  be- 
tween the  two  motor-coils,  and  by  reason  of  their  different  self- 


induction  a  difference  of  current-phase  is  obtained  that  starts  the 
motor  in  rotation.  In  starting,  the  field-coils  are  short  cir 
cuited. 

When  the  motor  has  attained  the  desired  speed,  the  switch  is 
shifted  to  the  position  shown  in  dotted  lines — that  is  to  say,  with 
the  levers  fg  resting  on  points  c  e.  This  connects  up  the  two 
armature  coils  in  series,  and  the  motor  will  then  run  as  a  syn- 
chronous motor.  The  field-coils  are  thrown  into  circuit  with  the 
direct  current  source  when  the  main  switch  is  shifted. 


CHAPTER  X. 

MOTOR  WITH  " CURRENT  LAG"  ARTIFICIALLY  SECURED. 

ONE  of  the  general  ways  followed  by  Mr.  Tesla  in  developing 
his  rotary  phase  motors  is  to  produce  practically  independent 
currents  differing  primarily  in  phase  and  to  pass  these  through  the 
motor-circuits.  Another  way  is  to  produce  a  single  alternating 
current,  to  divide  it  between  the  motor-circuits,  and  to  effect 
artificially  a  lag  in  one  of  these  circuits  or  branches,  as  by 
giving  to  the  circuits  different  self-inductive  capacity,  and  in 
other  ways.  In  the  former  case,  in  which  the  necessary  differ- 
ence of  phase  is  primarily  effected  in  the  generation  of  currents, 
in  some  instances,  the  currents  are  passed  through  the  energizing 
coils  of  both  elements  of  the  motor — the  field  and  armature ;  but 
a  further  result  or  modification  may  be  obtained  by  doing  this 
under  the  conditions  hereinafter  specified  in  the  case  of  motors 
in  which  the  lag,  as  above  stated,  is  artificially  secured. 

Figs.  42  to  4T,  inclusive,  are  diagrams  of  different  ways  in  which 
the  invention  is  carried  out ;  and  Fig.  48,  a  side  view  of  a  foam 
of  motor  used  by  Mr.  Tesla  for  this  purpose. 

A  B  in  Fig.  42  indicate  the  two  energizing  circuits  of  a  motor, 
and  c  D  two  circuits  on  the  armature.  Circuit  or  coil  A  is  con- 
nected in  series  with  circuit  or  coil  c,  and  the  two  circuits  B  D  are 
similarly  connected.  Between  coils  A  and  c  is  a  contact-ring  £, 
forming  one  terminal  of  the  latter,  and  a  brush  «,  forming  one 
terminal  of  the  former.  A  ring  d  and  brush  c  similarly  connect 
coils  B  and  D.  The  opposite  terminals  of  the  field-coils  connect 
to  one  binding  post  h  of  the  motor,  and  those  of  the  armature 
coils  are  similarly  connected  to  the  opposite  binding  post  i  through 
a  contact-ring  f  and  brush  g.  Thus  each  motor-circuit  while  in 
derivation  to  the  other  includes  one  armature  and  one  field  coil. 
These  circuits  are  of  different  self-induction,  and  may  be  made 
so  in  various  ways.  For  the  sake  of  clearness,  an  artificial  re- 
sistance R  is  shown  in  one  of  these  circuits,  and  in  the  other  a 
self-induction  coil  s.  When  an  alternating  current  is  passed 


POLYPHASE  CURRENTS. 


through  this  motor  it  divides  between  its  two  energizing-circuits. 
The  higher  self-induction  of  one  circuit  produces  a  greater  re- 
tardation or  lag  in  the  current  therein  than  in  the  other.  The 
difference  of  phase  between  the  two  currents  effects  the  rotation 
or  shifting  of  the  points  of  maximum  magnetic  effect  that  secures 


t — www — HM5&RJT — *& nffiMT — | 

^  l&t-*--* 


FIGS.  42,  43  and  44. 

the  rotation  of  the  armature.  In  certain  respects  this  plan  of  in- 
cluding both  armature  and  field  coils  in  circuit  is  a  marked  im- 
provement. Such  a  motor  has  a  good  torque  at  starting ;  yet  it 
has  also  considerable  tendency  to  synchronism,  owing  to  the  fact 


60  INVENTIONS  OF  NIKOLA  TE8LA. 

that  when  properly  constructed  the  maximum  magnetic  effects  in 
both  armature  and  field  coincide — a  condition  which  in  the  usual 
construction  of  these  motors  with  closed  armature  coils  is  not 
readily  attained.  The  motor  thus  constructed  exhibits  too,  a 
better  regulation  of  current  from  no  load  to  load,  and  there  is 
less  difference  between  the  apparent  and  real  energy  expended 
in  running  it.  The  true  synchronous  speed  of  this  form  of  motor 
is  that  of  the  generator  when  both  are  alike — that  is  to  say,  if 
the  number  of  the  coils  on  the  armature  and  on  the  field  is  a?,  the 
motor  will  run  normally  at  the  same  speed  as  a  generator  driving 


Uv^-^Mfa^  Lum' 


Fms.  45,  46  and  47. 

it  if  the  number  of  field  magnets  or  poles  of  the  same  be  also  or. 

Fig.  43  shows  a  somewhat  modified  arrangement  of  circuits. 
There  is  in  this  case  but  one  armature  coil  E,  the  winding  of 
which  maintains  effects  corresponding  to  the  resultant  poles  pro- 
duced by  the  two  field-circuits. 

Fig.  44  represents  a  disposition  in  which  both  armature  and 
field  are  wound  with  two  sets  of  coils,  all  in  multiple  arc  to  the 
line  or  main  circuit.  The  armature  coils  are  wound  to  corre- 
spond with  the  field-coils  with  respect  to  their  self-induction.  A 
modification  of  this  plan  is  shown  in  Fig.  45 — that  is  to  say,  the 


POLYPHASE  CURRENTS 


61 


two  field  coils  and  two  armature  coils  are  in  derivation  to  them- 
selves and  in  series  with  one  another.  The  armature  coils  in 
this  case,  as  in  the  previous  figure,  are  wound  for  different  self- 
induction  to  correspond  with  the  field  coils. 

Another  modification  is  shown  in  Fig.  46.  In  this  case  only 
one  armature-coil,  as  D,  is  included  in  the  line-circuit,  while  the 
other,  as  c,  is  short-circuited. 

In  such  a  disposition  as  that  shown  in  Fig.  43,  or  where  only 
one  armature-coil  is  employed,  the  torque  on  the  start  is  some- 
what reduced,  while  the  tendency  to  synchronism  is  somewhat 


FIG.  48. 

increased.  In  such  a  disposition  as  shown  in  Fig.  4H,  the  oppo- 
site conditions  would  exist.  In  both  instances,  however,  there 
is  the  advantage  of  dispensing  with  one  contact-ring. 

In  Fig.  4(5  the  two  field-coils  and  the  armature-coil  D  are  in 
multiple  arc.  In  Fig.  47  this  disposition  is  modified,  coil  D  be- 
ing shown  in  series  with  the  two  field-coils. 

Fig.  48  is  an  outline  of  the  general  form  of  motor  in  which 
this  invention  is  embodied.  The  circuit  connections  between 
the  armature  and  field  coils  are  made,  as  indicated  in  the  previ- 
ous figures,  through  brushes  and  rings,  which  are  not  shown. 


CHAPTER   XI. 

ANOTHER  METHOD  OF  TRANSFORMATION  FROM   A    TORQUE  TO  A 
SYNCHRONIZING  MOTOR. 

IN  a  preceding  chapter  we  have  described  a  method  by  which 
Mr.  Tesla  accomplishes  the  change  in  his  type  of  rotating  field 
motor  from  a  torque  to  a  synchronizing  motor.  As  will  be  ob- 
served, the  desired  end  is  there  reached  by  a  change  in  the  cir- 
cuit connections  at  the  proper  moment.  We  will  now  proceed 
to  describe  another  way  of  bringing  about  the  same  result.  The 
principle  involved  in  this  method  is  as  follows : — 

If  an  alternating  current  be  passed  through  the  field  coils  only 
of  a  motor  having  two  energizing  circuits  of  different  self-induc- 
tion and  the  armature  coils  be  short-circuited,  the  motor  will  have 
a  strong  torque,  but  little  or  no  tendency  to  synchronism  with 
the  generator ;  but  if  the  same  current  which  energizes  the  field 
be  passed  also  through  the  armature  coils  the  tendency  to  remain 
in  synchronism  is  very  considerably  increased.  This  is  due  to 
the  fact  that  the  maximum  magnetic  effects  produced  in  the  field 
and  armature  more  nearly  coincide.  On  this  principle  Mr. 
Tesla  constructs  a  motor  having  independent  field  circuits  of 
different  self-induction,  which  are  joined  in  derivation  to  a 
source  of  alternating  currents.  The  armature  is  wound  with  one 
or  more  coils,  which  are  connected  with  the  field  coils  through 
contact  rings  and  brushes,  and  around  the  armature  coils  a  shunt 
is  arranged  with  means  for  opening  or  closing  the  same.  In  start- 
ing this  motor  the  shunt  is  closed  around  the  armature  coils, 
which  will  therefore  be  in  closed  circuit.  When  the  current  is 
directed  through  the  motor,  it  divides  between  the  two  circuits, 
(it  is  not  necessary  to  consider  any  case  where  there  are  more 
than  two  circuits  used),  which,  by  reason  of  their  different  self- 
induction,  secure  a  difference  of  phase  between  the  two  currents 
in  the  two  branches,  that  produces  a  shifting  or  rotation  of  the 
of  the  poles.  By  the  alternations  of  current,  other  currents  are 
induced  in  the  closed — or  short-circuited — armature  coils  and  the 


POLYPHASE  CURRENTS. 


63 


motor  has  a  strong  torque.  When  the  desired  speed  is  reached, 
the  shunt  around  the  armature-coils  is  opened  and  the  current 
directed  through  both  armature  and  field  coils.  Under  these 
conditions  the  motor  has  a  strong  tendency  to  synchronism. 

In  Fig.  49,  A  and  B  designate  the  field  coils  of  the  motor.  As 
the  circuits  including  these  coils  are  of  different  self-induction, 
this  is  represented  by  a  resistance  coil  R  in  circuit  with  A,  and  a 


FKJS.  49(  50  and  51. 


self-induction  coil  s  in  circuit  with  B.  The  same  result  may  of 
course  be  secured  by  the  winding  of  the  coils,  c  is  the  armature 
circuit,  the  terminals  of  which  are  rings  a  J.  Brushes  c  d  bear 
on  these  rings  and  connect  with  the  line  and  field  circuits.  D  is 
the  shunt  or  short  circuit  around  the  armature.  E  is  the  switch 
in  the  shunt. 

It  will  be  observed  that  in  such  a  disposition  as  is  illustrated  in 


•vi 


INVENTIONS  OF  NIKOLA  TESLA. 


Fig.  49,  the  field  circuits  A  and  B  being  of  different  self-induction, 
there  will  always  be  a  greater  lag  of  the  current  in  one  than  the 
other,  and  that,  generally,  the  armature  phases  will  not  corre- 
spond with  either,  but  with  the  resultant  of  both.  It  is  therefore 
important  to  observe  the  proper  rule  in  winding  the  armature. 
For  instance,  if  the  motor  have  eight  poles — four  in  each  circuit 
— there  will  be  four  resultant  poles,  and  hence  the  armature 
winding  should  be  such  as  to  produce  four  poles,  in  order  to  con- 
stitute a  true  synchronizing  motor. 

The  diagram,  Fig.  50,  differs  from  the  previous  one  only  in 
respect  to  the  order  of  connections.  In  the  present  case  the  arm- 
ature-coil, instead  of  being  in  series  with  the  field-coils,  is  in  mul- 
tiple arc  therewith.  The  armature- winding  may  be  similar  to 
that  of  the  field — that  is  to  say,  the  armature  may  have  two  or 
more  coils  wound  or  adapted  for  different  self-induction  and 


FIG.  52. 

adapted,  preferably,  to  produce  the  same  difference  of 
phase  as  the  field-coils.  On  starting  the  motor  the  shunt 
is  closed  around  both  coils.  This  is  shown  in  Fig.  51,  in 
which  the  armature  coils  are  K  <;.  To  indicate  their  different 
electrical,  character,  there  are  shown  in  circuit  with  them,  respect- 
ively, the  resistance  R'  and  the  self-induction  coil  s'.  The  two 
armature  coils  are  in  series  with  the  field-coils  and  the  same  dis- 
position of  the  shunt  or  short-circuit  u  is  used.  It  is  of  advan- 
tage in  the  operation  of  motors  of  this  kind  to  construct  or  wind 
the  armature  in  such  manner  that  when  short-circuited  on  the 
start  it  will  have  a  tendency  to  reach  a  higher  speed  than  that 
which  synchronizes  with  the  generator.  For  example,  a  given 
motor  having,  say,  eight  poles  should  run,  with  the  armature  coil 
short-circuited,  at  two  thousand  revolutions  per  minute  to  bring 
it  up  to  synchronism.  It  will  generally  happen,  however,  tha't 


POLYPHASE  CURRENTS.  65 

this  speed  is  not  reached,  owing  to  the  fact  that  the  armature 
and  field  currents  do  not  properly  correspond,  so  that  when  the 
current  is  passed  through  the  armature  (the  motor  not  being 
quite  up  to  synchronism)  there  is  a  liability  that  it  will  not  "hold 
on,"  as  it  is  termed.  It  is  preferable,  therefore,  to  so  wind  or 
construct  the  motor  that  on  the  start,  when  the  armature  coils 
are  short-circuited,  the  motor  will  tend  to  reach  a  speed  higher 
than  the  synchronous — as  for  instance,  double  the  latter.  In 
such  case  the  difficulty  above  alluded  to  is  not  felt,  for  the  mo- 
tor will  always  hold  up  to  synchronism  if  the  synchronous  speed — 
in  the  case  supposed  of  two  thousand  revolutions — is  reached  or 
passed.  This  may  be  accomplished  in  various  ways ;  but  for  all 
practical  purposes  the  following  will  suffice :  On  the  armature 
are  wound  two  sets  of  coils.  At  the  start  only  one  of  these  is 


Fm.  53. 

short-circuited,  thereby  producing  a  number  of  poles  on  the  ar- 
mature, which  will  tend  to  run  the  speed  up  above  the  synchron- 
ous limit.  When  such  limit  is  reached  or  passed,  the  current  is 
directed  through  the  other  coil,  which,  by  increasing  the  number 
<>f  armature  poles,  tends  to  maintain  synchronism. 

In  Fig.  52,  such  a  disposition  is  shown.  The  motor  having, 
say,  eight  poles  contains  two  field-circuits  A  and  B,  of  different 
self-induction.  The  armature  has  two  coils  F  and  G.  The  former 
is  closed  upon  itself,  the  latter  connected  with  the  field  and  line 
through  contact-rings  a  5,  brushes  G  d,  and  a  switch  K.  On  the 
start  the  coil  F  alone  is  active  and  the  motor  tends  to  run  at  a 
speed  above  the  synchronous;  but  when  the  coil  G  is  connected 
to  the  circuit  the  number  of  armature  poles  is  increased,  while 
the  motor  is  made  a  true  synchronous  motor.  This  disposition 


66  INVENTIONS  OF  NIKOLA  TESLA. 

has  the  advantage  that  the  closed  armature-circuit  imparts  to  the 
motor  torque  when  the  speed  falls  off,  but  at  the  same  time  the 
conditions  are  such  that  the  motor  comes  out  of  synchronism 
more  readily.  To  increase  the  tendency  to  synchronism,  two 
circuits  may  be  used  on  the  armature,  one  of  which  is  short-cir- 
cuited on  the  start  and  both  connected  with  the  external  circuit 
after  the  synchronous  speed  is  reached  or  passed.  This  disposi- 
tion is  shown  in  Fig.  53.  There  are  three  contact-rings  a  b  e 
and  three  brushes  c  d  f,  which  connect  the  armature  circuits 
with  the  external  circuit.  ( )n  starting,  the  switch  H  is  turned  to 
complete  the  connection  between  one  binding-post  p  and  the  field- 
coils.  This  short-circuits  one  of  the  armature-coils,  as  G.  The 
other  coil  F  is  out  of  circuit  and  open.  When  the  motor  is  up 
to  speed,  the  switch  H  is  turned  back,  so  that  the  connection 
from  binding-post  p  to  the  field  coils  is  through  the  coil  G,  and 
switch  K  is  closed,  thereby  including  coil  F  in  multiple  arc  with 
the  field  coils.  Both  armature  coils  arethus  active. 

From  the  above-described  instances  it  is  evident  that  many 
other  dispositions  for  carrying  out  the  invention  are  possible. 


CHAPTER  XII. 

"  MAGNETIC  LAG  "  MOTOK. 

THE  following  description  deals  with  another  form  of  motor, 
namely,  depending  on  "  magnetic  lag  "  or  hysteresis,  its  peculiar- 
ity being  that  in  it  the  attractive  effects  or  phases  while  lagging 
behind  the  phases  of  current  which  produce  them,  are  mani- 
fested simultaneously  and  not  successively.  The  phenomenon 
utilized  thus  at  an  early  stage  by  Mr.  Tesla,  was  not  generally 
believed  in  by  scientific  men,  and  Prof.  Ayrton  was  probably 
iirst  to  advocate  it  or  to  elucidate  the  reason  of  its  supposed  ex- 
istence. 

Fig.  54-  is  a  side  view  of  the  motor,  in  elevation.  Fig.  55  is 
a  part-sectional  view  at  right  angles  to  Fig.  54.  Fig.  56  is  an 
end  viewT  in  elevation  and  part  section  of  a  modification,  and 
Fig.  57  is  a  similar  view  of  another  modification. 

In  Figs.  54  and  55,  A  designates  a  base  or  stand,  and  B  B 
the  supporting-frame  of  the  motor.  Bolted  to  the  supporting- 
frame  are  two  magnetic  cores  or  pole-pieces  c  c',  of  iron  or 
soft  steel.  These  may  be  subdivided  or  laminated,  in  which 
case  hard  iron  or  steel  plates  or  bars  should  be  used,  or  they 
should  be  wound  with  closed  coils.  D  is  a  circular  disc  arma- 
ture, built  up  of  sections  or  plates  of  iron  and  mounted  in  the 
frame  between  the  pole-pieces  c  c',  curved  to  conform  to  the 
circular  shape  thereof.  This  disc  may  be  wound  with  a  number 
of  closed  coils  E.  v  F  are  the  main  energizing  coils,  supported 
by  the  supporting-frame,  so  as  to  include  within  their  magnet- 
izing influence  both  the  pole-pieces  c  c'  and  the  armature  i>. 
The  pole-pieces  c  c'  project  out  beyond  the  coils  F  F  on  op- 
posite sides,  as  indicated  in  the  drawings.  If  an  alternating 
current  be  passed  through  the  coils  F  F,  rotation  of  the  arma- 
ture will  be  produced,  and  this  rotation  is  explained  by  the 
following  apparent  action,  or  mode  of  operation :  An  impulse 
of  current  in  the  coils  F  F  establishes  two  polarities  in  the  mo- 
tor. The  protruding  end  of  pole-piece  c,  for  instance,  will  be 


68  INVENTIONS  OF  NIKOLA  TE8LA. 

of  one  sign,  and  the  corresponding  end  of  pole-piece  c  will  be 
of  the  opposite  sign.  The  armature  also  exhibits  two  poles. at 
right  angles  to  the  coils  r  F,  like  poles  to  those  in  the  pole- 
pieces  being  011  the  same  side  of  the  coils.  While  the  current 
is  flowing  there  is  no  appreciable  tendency  to  rotation  devel- 
oped ;  but  after  each  current  impulse  ceases  or  begins  to  fall, 
the  magnetism  in  the  armature  and  in  the  ends  of  the  pole- 
pieces  c  c'  lags  or  continues  to  manifest  itself,  which  produces  a 
rotation  of  the  armature  by  the  repellent  force  between  the 
more  closely  approximating  points  of  maximum  magnetic  effect. 
This  effect  is  continued  by  the  reversal  of  current,  the  polari- 
ties of  field  and  armature  being  simply  reversed.  One  or  both 
of  the  elements — the  armature  or  field — may  be  wound  with 


FIG.  54 


closed  induced  coils  to  intensify  this  effect.  Although  in  the 
illustrations  but  one  of  the  fields  is  shown,  each  element  of  the 
motor  really  constitutes  a  field,  wound  with  the  closed  coils, 
the  currents  being  induced  mainly  in  those  convolutions  or  coils 
which  are  parallel  to  the  coils  r  F. 

A  modified  form  of  this  motor  is  shown  in  Fig.  5(5.  In  this 
form  G  is  one  of  two  standards  that  support  the  bearings  for 
the  armature-shaft.  H  H  are  uprights  or  sides  of  a  frame,  prefer- 
ably magnetic,  the  ends  c  c'  of  which  are  bent  in  the  manner 
indicated,  to  conform  to  the  shape  of  the  armature  D  and  form 
field-magnet  poles.  The  construction  of  the  armature  may  be 
the  same  as  in  the  previous  figure,  or  it  may  be  simply  a  mag- 
netic disc  or  cylinder,  as  shown,  and  a  coil  or  coils  F  F  are  se- 


POLYPHASE  CURRENT*. 


69 


cured  in  position  to  surround  both  the  armature  and  the  poles 
c  c'.  The  armature  is  detachable  from  its  shaft,  the  latter  being 
passed  through  the  armature  after  it  has  been  inserted  in  posi- 
tion. The  operation  of  this  form  of  motor  is  the  same  in  prin- 
ciple as  that  previously  described  and  needs  no  further  explana- 
tion. 

One  of  the  most  important  features  in  alternating  current 
motors  is,  however,  that  they  should  be  adapted  to  and  capable 
of  running  efficiently  on  the  alternating  circuits  in  present  use, 
in  which  almost  without  exception  the  generators  yield  a  very 
high  number  of  alternations.  Such  a  motor,  of  the  type  under 
consideration,  Mr.  Tesla  has  designed  by  a  development  of  the 
principle  of  the  motor  shown  in  Fig.  56,  making  a  multipolar 
motor,  which  is  illustrated  in  Fig.  57.  In  the  construction  of 


FIG.  56. 


FIG.  57. 


this  motor  he  employs  an  annular  magnetic  frame  j,  with  in- 
wardly-extending ribs  or  projections  K,  the  ends  of  which  all 
bend  or  turn  in  one  direction  and  are  generally  shaped  to  con- 
form to  the  curved  surface  of  the  armature.  Coils  F  F  are  wound 
from  one  part  K  to  the  one  next  adjacent,  the  ends  or  loops  of 
each  coil  or  group  of  wires  being  carried  over  toward  the  shaft, 
so  as  to  form  y -shaped  groups  of  convolutions  at  each  end  of  the 
armature.  The  pole-pieces  C  C',  being  substantially  concentric 
with  the  armature,  form  ledges,  along  which  the  coils  are  laid 
and  should  project  to  some  extent  beyond  the  the  coils,  as  shown. 
The  cylindrical  or  drum  armature  D  is  of  the  same  construction 
as  in  the  other  motors  described,  and  is  mounted  to  rotate  within 
the  annular  frame  j  and  1  Jet  ween  the  U-shaped  ends  or  bends  of 


70  INVENTIONS  OF  NIKOLA   TE8LA. 

the  coils  F.  The  coils  F  are  connected  in  multiple  or  in  series 
with  a  source  of  alternating  currents,  and  are  so  wound  that 
with  a  current  or  current  impulse  of  given  direction  they  will 
make  the  alternate  pole-pieces  c  of  one  polarity  and  the  other 
pole-pieces  c'  of  the  opposite  polarity.  The  principle  of  the 
operation  of  this  motor  is  the  same  as  the  other  above  de- 
scribed, for,  considering  any  two  pole-pieces  c  c',  a  current 
impulse  passing  in  the  coil  which  bridges  them  or  is  wound 
over  both  tends  to  establish  polarities  in  their  ends  of  opposite 
sign  and  to  set  up  in  the  armature  core  between  them  a  polarity 
of  the  same  sign  as  that  of  the  nearest  pole-piece  c.  Upon  the 
fall  or  cessation  of  the  current  impulse  that  established  these 
polarities  the  magnetism  which  lags  behind  the  current  phase, 
and  which  continues  to  manifest  itself  in  the  polar  projections 
c  c'  and  the  armature,  produces  by  repulsion  a  rotation  of  the 
armature.  The  effect  is  continued  by  each  reversal  of  the  cur- 
rent. What  occurs  in  the  case  of  one  pair  of  pole-pieces  occurs 
simultaneously  in  all,  so  that  the  tendency  to  rotation  of  the 
armature  is  measured  by  the  sum  of  all  the  forces  exerted  by  the 
pole-pieces,  as  above  described.  In  this  motor  also  the  mag- 
netic lag  or  effect  is  intensified  by  winding  one  or  both  cores 
with  closed  induced 'coils.  The  armature  core  is  shown  as  thus 
wound.  When  closed  coils  are  used,  the  cores  should  be  lamin- 
ated. 

It  is  evident  that  a  pulsatory  as  well  as  an  alternating  current 
might  be  used  to  drive  or  operate  the  motors  above  described. 

It  will  be  understood  that  the  degree  of  subdivision,  the  mass 
of  the  iron  in  the  cores,  their  size  and  the  number  of  alternations 
in  the  current  employed  to  run  the  motor,  must  be  taken  into 
consideration  in  order  to  properly  construct  this  motor.  In  other 
words,  in  all  such  motors  the  proper  relations  between  the  num- 
ber of  alternations  and  the  mass,  size,  or  quality  of  the  iron  must 
be  preserved  in  order  to  secure  the  best  results. 


CHAPTEE  XIII. 

METHOD  OF  OBTAINING  DIFFERENCE  OF  PHASE  BY  MAGNETIC 
SHIELDING. 

IN  that  class  of  motors  in  which  two  or  more  sets  of  energizing 
magnets  are  employed,  and  in  which  by  artificial  means  a  certain 
interval  of  time  is  made  to  elapse  between  the  respective  max- 
imum or  minimum  periods  or  phases  of  their  magnetic  attraction 
or  effect,  the  interval  or  difference  in  phase  between  the  two  sets 
of  magnets  is  limited  in  extent.  It  is  desirable,  however,  for  the 
economical  working  of  such  motors  that  the  strength  or  attraction 
of  one  set  of  magnets  should  be  maximum,  at  the  time  when  that 
of  the  other  set  is  minimum,  and  conversely  ;  but  these  conditions 
have  not  heretofore  been  realized  except  in  cases  where  the  two 
currents  have  been  obtained  from  independent  sources  in  the 
same  or  different  machines.  Mr.  Tesla  has  therefore  devised  a 
motor  embodying  conditions  that  approach  more  nearly  the  theo- 
retical requirements  of  perfect  working,  or  in  other  words,  he 
produces  artificially  a  difference  of  magnetic  phase  by  means  of 
a  current  from  a  single  primary  source  sufficient  in  extent  to 
meet  the  requirements  of  practical  and  economical  working.  He 
employs  a  motor  with  two  sets  of  energizing  or  field  magnets, 
each  wound  with  coils  connected  with  a  source  of  alternating  or 
rapidly-varying  currents,  but  forming  two  separate  paths  or 
circuits.  The  magnets  of  one  set  are  protected  to  a  certain  ex- 
tent from  the  energizing  action  of  the  current  by  means  of  a 
magnetic  shield  or  screen  interposed  between  the  magnet  and  its 
energizing  coil.  This  shield  is  properly  adapted  to  the  conditions 
of  particular  cases,  so  as  to  shield  or  protect  the  main  core  from 
magnetization  until  it  has  become  itself  saturated  and  no  longer 
capable  of  containing  all  the  lines  of  force  produced  by  the  cur- 
rent. It  will  be  seen  that  by  this  means  the  energizing  action 
begins  in  the  protected  set  of  magnets  a  certain  arbitrarily- 
determined  period  of  time  later  than  in  the  other,  and  that  by 
this  means  alone  or  in  conjunction  with  other  means  or  devices 


72  INVENTIONS  OF  NIKOLA  TESLA. 

heretofore  employed  a  practical  difference  of  magnetic  phase 
may  readily  be  secured. 

Fig.  58  is  a  view  of  a  motor,  partly  in  section,  with  a  dia- 
gram illustrating  the  invention.  Fig.  59  is  a  similar  view  of  a 
modification  of  the  same. 

In  Fig.  58,  which  exhibits  the  simplest  form  of  the  invention, 
A  A  is  the  field-magnet  of  a  motor,  having,  say,  eight  poles  or 
inwardly-projecting  cores  B  and  c.  The  cores  B  form  one  set  of 
magnets  and  are  energized  by  coils  D.  The  cores  c,  forming 
the  other  set  are  energized  by  coils  E,  and  the  coils  are 
connected,  preferablv,  in  series  with  one  another,  in  two  de- 
rived or  branched  circuits,  r  o,  respectively,  from  a  suitable 
source  of  current.  Each  coil  E  is  surrounded  by  a  magnetic 
shield  n,  which  is  preferably  composed  of  an  annealed,  insulated, 


FIG.  58. 


FIG.  59. 


or  oxidized  iron  wire  wrapped  or  wound  on  the  coils  in  the  man- 
ner indicated  so  as  to  form  a  closed  magnetic  circuit  around  the 
coils  and  between  the  same  and  the  magnetic  cores  c.  Be- 
tween the  pole  pieces  or  cores  B  c  is  mounted  the-  armature  K, 
which,  as  is  usual  in  this  type  of  machines,  is  wound  with  coils 
L  closed  upon  themselves.  The  operation  resulting  from  this 
disposition  is  as  follows:  If  a  current  impulse  be  directed 
through  the  two  circuits  of  the  motor,  it  will  quickly  energize 
the  cores  B,  but  not  so  the  cores  c,  for  the  reason  that  in 
passing  through  the  coils  E  there  is  encountered  the  influence 
of  the  closed  magnetic  circuits  formed  by  the  shields  H.  The 
first  effect  is  to  retard  effectively  the  current  impulse  in  circuit 
G,  while  at  the  same  time  the  proportion  of  current  which  does 
pass  does  not  magnetize  the  cores  c,  which  are  shielded  or 


POLYPHASE  CURRENTS.  73 

screened  by  the  shields  H.  As  the  increasing  electromotive 
force  then  urges  more  current  through  the  coils  E,  the  iron  wire 
H  becomes  magnetically  saturated  and  incapable  of  carrying  all 
the  lines  of  force,  and  hence  ceases  to  protect  the  cores  c,  which 
becomes  magnetized,  developing  their  maximum  effect  after  an 
interval  of  time  subsequent  to  the  similar  manifestation  of  strength 
in  the  other  set  of  magnets,  the  extent  of  which  is  arbitrarily 
determined  by  the  thickness  of  the  shield  H,  and  other  well-un- 
derstood conditions. 

From  the  above  it  will  be  seen  that  the  apparatus  or  device 
acts  in  two  ways.  First,  by  retarding  the  current,  and,  second, 
by  retarding  the  magnetization  of  one  set  of  the  cores,  from 
which  its  effectiveness  will  readily  appear. 

Many  modifications  of  the  principle  of  this  invention  are  pos- 
sible. One  useful  and  efficient  application  of  the  invention  is 
shown  in  Fig.  59.  In  this  figure  a  motor  is  shown  similar  in  all 
respects  to  that  above  described,  except  that  the  iron  wire  H,  which 
is  wrapped  around  the  coils  E,  is  in  this  case  connected  in  series 
with  the  coils  D.  The  iron-wire  coils  H,  are  connected  and  wound, 
so  as  to  have  little  or  no  self-induction,  and  being  added  to  the 
resistance  of  the  circuit  F,  the  action  of  the  current  in  that  cir- 
cuit will  be  accelerated,  while  in  the  other  circuit  G  it  will  be 
retarded.  The  shield  H  may  be  made  in  many  forms,  as  will  be 
understood,  and  used  in  different  ways,  as  appears  from  the 
foregoing  description. 

As  a  modification  of  his  type  of  motor  with  "  shielded  "  fields^ 
Mr.  Tesla  has  constructed  a  motor  with  a  field-magnet  having 
two  sets  of  poles  or  inwardly-projecting  cores  and  placed  side 
Uy  side,  so  as  practically  to  form  two  fields  of  force  and  alter- 
nately disposed — that  is  to  say,  with  the  poles  of  one  set  or  field 
opposite  the  spaces  between  the  other.  He  then  connects  the  free 
ends  of  one  set  of  poles  by  means  of  laminated  iron  bands  or 
bridge-pieces  of  considerably  smaller  cross-section  than  the  cores 
themselves,  whereby  the  cores  will  all  form  parts  of  complete 
magnetic  circuits.  When  the  coils  on  each  set  of  magnets  are 
connected  in  multiple  circuits  or  branches  from  a  source  of  al- 
ternating currents,  electromotive  forces  are  set  up  in  or  im- 
pressed upon  each  circuit  simultaneously ;  but  the  coils  on  the 
magnetically  bri'dged  or  shunted  cores  will  have,  by  reason  of 
the  -closed  magnetic-circuits,  a  high  self-induction,  which  retards 
the  current,  permitting  at  the  beginning  of  each  impulse  but  lit- 


7-1 


INVENTIONS  OF  NIKOLA  TESLA. 


tie  current  to  pass.  On  the  other  hand,  no  such  opposition  being 
encountered  in  the  other  set  of  coils,  the  current  passes  freely 
through  them,  magnetizing  the  poles  on  which  they  are  wound. 
As  soon,  however,  as  the  laminated  bridges  become  saturated 
and  incapable  of  carrying  all  the  lines  of  force  which  the  rising 
electromotive  force,  and  consequently  increased  current,  pro- 
duce, free  poles  are  developed  at  the  ends  of  the  cores,  which, 
acting  in  conjunction  with  the  others,  produce  rotation  of  the 
armature. 

The  construction  in  detail  by  which  this  invention  is  illustrated 
is  shown  in  the  accompanying  drawings. 

Fig.  60  is  a  view  in  side  elevation  of  a  motor  embodying  the 
principle.  Fig.  61  is  a  vertical  cross-section  of  the  motor.  A  is 
the  frame  of  the  motor,  which  should  be  built  up  of  sheets  of 
iron  punched  out  to  the  desired  shape  and  bolted  together  witli 


FIG.  60. 


FIG.  61. 


insulation  between  the  sheets.  When  complete,  the  frame  makes 
a  field-magnet  with  inwardly  projecting  pole-pieces  B  and  c.  To 
adapt  them  to  the  requirements  of  this  particular  case  these  pole- 
pieces  are  out  of  line  with  one  another,  those  marked  B  surround- 
ing one  end  of  the  armature  and  the  others,  as  c,  the  opposite 
end,  and  they  are  disposed  alternately — that  is  to  say,  the  pole- 
pieces  of  one  set  occur  in  line  with  the  spaces  between  those  of  the 
other  sets. 

The  armature  D  is  of  cylindrical  form,  and  is  also  laminated  in 
the 'usual  way  and  is  wound  longitudinally  with  coils  closed  upon 
themselves.  The  pole-pieces  c  are  connected  or  shunted  by 
bridge-pieces  E.  These  may  be  made  independently  and  attached 
to  the  pole-pieces,  or  they  may  be  parts  of  the  forms  or  blanks 
stamped  or  punched  out  of  sheet-iron.  Their  size  or  mass  is  de- 


POLYPHASE  CURRENTS.  75 

termined  by  various  conditions,  such  as  the  strength  of  the  cur- 
rent to  be  employed,  the  mass  or  size  of  the  cores  to  which  they 
are  applied,  and  other  familiar  conditions. 

Coils  F  surround  the  pole-pieces  B,  and  other  coils  G  are  wound 
on  the  pole-pieces  c.  These  coils  are  connected  in  series  in  two 
circuits,  which  are  branches  of  a  circuit  from  a  generator  of  alter- 
nating currents,  and  they  may  be  so  wound,  or  the  respective 
circuits  in  which  they  are  included  may  be  so  arranged,  that  the 
circuit  of  coils  G  will  have,  independently  of  the  particular  con- 
struction described,  a  higher  self-induction  than  the  other  circuit 
or  branch. 

The  function  of  the  shunts  or  bridges  E  is  that  they  shall  form 
with  the  cores  c  a  closed  magnetic  circuit  for  a  current  up  to  a 
predetermined  strength,  so  that  when  saturated  by  such  current 
and  unable  to  carry  more  lines  of  force  than  such  a  current  pro- 
duces they  will  to  no  further  appreciable  extent  interfere  with 
the  development,  by  a  stronger  current,  of  free  magnetic  poles  at 
the  ends  of  the  cores  c. 

In  such  a  motor  the  current  is  so  retarded  in  the  coils  G,  and 
the  manifestation  of  the  free  magnetism  in  the  poles  c  is  so  delayed 
beyond  the  period  of  maximum  magnetic  effect  in  poles  B,  that  a 
strong  torque  is  produced  and  the  motor  operates  with  approx- 
imately the  power  developed  in  a  motor  of  this  kind  energized 
by  independently  generated  currents  differing  by  a  full  quarter 
phase. 


CHAPTEK  XIV. 
TYPE  OF  TESLA  SINGLE-PHASE  MOTOR. 

UP  TO  this  point,  two  principal  types  of  Tesla  motors  have 
been  described :  First,  those  containing  two  or  more  energizing 
circuits  through  which  are  caused  to  pass  alternating  currents 
differing  from  one  another  in  phase  to  an  extent  sufficient  to 
produce  a  continuous  progression  or  shifting  of  the  poles  or 
points  of  greatest  magnetic  eifect,  in  obedience  to  which  the 
movable  element  of  the  motor  is  maintained  in  rotation ;  second, 
those  containing  poles,  or  parts  of  different  magnetic  suscepti- 
bility, which  under  the  energizing  influence  of  the  same  current 
or  two  currents  coinciding  in  phase  will  exhibit  differences  in 
their  magnetic  periods  or  phases.  In  the  first  class  of  motors 
the  torque  is  due  to  the  magnetism  established  in  different  por- 
tions of  the  motor  by  currents  from  the  same  or  from  inde- 
pendent sources,  and  exhibiting  time  differences  in  phase.  In 
the  second  class  the  torque  results  from  the  energizing  effects  of 
a  current  upon  different  parts  of  the  motor  which  differ  in  mag- 
netic susceptibility — in  other  words,  parts  which  respond  in  the 
same  relative  degree  to  the  action  of  a  current,  not  simultaneously, 
but  after  different  intervals  of  time. 

In  another  Tesla  motor,  however,  the  torque,  instead  of  being 
solely  the  result  of  a  tjme  difference  in  the  magnetic  periods  or 
phases  of  the  poles  or  attractive  parts  to  whatever  cause  due,  is 
produced  by  an  angular  displacement  of  the  parts  which,  though 
movable  with  respect  to  one  another,  are  magnetized  simultane- 
ously, or  approximately  so,  by  the  same  currents.  This  principle 
of  operation  has  been  embodied  practically  in  a  motor  in  which 
the  necessary  angular  displacement  between  the  points  of  greatest 
magnetic  attraction  in  the  two  elements  of  the  motor — the  arma- 
ture and  field — is  obtained  by  the  direction  of  the  lamination  of 
the  magnetic  cores  of  the  elements. 

Fig.  62  is  a  side  view  of  such  a  motor  with  a  portion  of  its 
armature  core  exposed.  Fig.  63  is  an  end  or  edge  view  of  the 


POLYPHASE  CURRENTS. 


77 


same.     Fig.  64  is  a  central  cross-section  of  the  same,  the  arma- 
ture being  shown  mainly  in  elevation. 

Let  A  A  designate  two  plates  built  up  of  thin  sections  or 
laminae  of  soft  iron  insulated  more  or  less  from  one  another  and 
held  together  by  bolts  a  and  secured  to  a  base  B.  The  inner 
faces  of  these  plates  contain  recesses  or  grooves  in  which  a  coil 
or  coils  D  are  secured  obliquely  to  the  direction  of  the  lamina- 
tions. Within  the  coils  D  is  a  disc  E,  preferably  composed  of 
a  spirally- wound  iron  wire  or  ribbon  or  a  series  of  concentric- 
rings  and  mounted  on  a  shaft  r,  having  bearings  in  the  plates 
A  A.  Such  a  device  when  acted  upon  by  an  alternating  current 
is  capable  of  rotation  and  constitutes  a  motor,  the  operation  of 
which  may  be  explained  in  the  following  manner :  A  current  or 
current-impulse  traversing  the  coils  n  tends  to  magnetize  the 


FIG.  62. 


FIG.  63. 


FIG.  64. 


cores  A  A  and  E,  all  of  which  are  within  the  influence  of  the 
lield  of  the  coils.  The  poles  thus  established  would  naturally 
lie  in  the  same  line  at  right  angles  to  the  coils  D,  but  in  the 
plates  A  they  are  deflected  by  reason  of  the  direction  of  the 
laminations,  and  appear  at  or  near  the  extremities  of  these  plates. 
In  the  disc,  however,  where  these  conditions  are  not  present,  the 
poles  or  points  of  greatest  attraction  are  on  a  line  at  right 
angles  to  the  plane  of  the  coils;  hence  there  will  be  a  torque  es- 
tablished by  this  angular  displacement  of  the  poles  or  magnetic 
lines,  which  starts  the  disc  in  rotation,  the  magnetic  lines  of  the 
armature  and  field  tending  toward  a  position  of  parallelism. 
This  rotation  is  continued  and  maintained  by  the  reversals  of 
the  current  in  coils  D  D,  which  change  alternately  the  polarity  of 
the  field-cores  A  A.  This  rotary  tendency  or  effect  will  be  greatly 


78  INVENTIONS  OF  NIKOLA  TESLA. 

increased  by  winding  the  disc  with  conductors  G,  closed  upon 
themselves  and  having  a  radial  direction,  whereby  the  magnetic 
intensity  of  the  poles  of  the  disc  will  be  greatly  increased  by 
the  energizing  effect  of  the  currents  induced  in  the  coils  G  by  the 
alternating  currents  in  coils  D. 

The  cores  of  the  disc  and  field  may  or  may  not  be  of  different 
magnetic  susceptibility — that  is  to  say,  they  may  both  be  of  the 
same  kind  of  iron,  so  as  to  be  magnetized  at  approximately  the 
same  instant  by  the  coils  D;  or  one  may  be  of  soft  iron  and  the 
other  of  hard,  in  order  that  a  certain  time  may  elapse  between 
the  periods  of  their  magnetization.  In  either  case  rotation  will 
be  produced ;  but  unless  the  disc  is  provided  with  the  closed  en- 
ergizing coils  it  is  desirable  that  the  above-described  difference  of 
magnetic  susceptibility  be  utilized  to  assist  in  its  rotation. 

The  cores  of  the  field  and  armature  may  be  made  in  various 
ways,  as  will  be  well  understood,  it  being  only  requisite  that  the 
laminations  in  each  be  in  such  direction  as  to  secure  the  neces- 
sary angular  displacement  of  the  points  of  greatest  attraction. 
Moreover,  since  the  disc  may  be  considered  as  made  up  of  an 
infinite  number  of  radial  arms,  it  is  obvious  that  what  is  true  of 
a  disc  holds  for  many  other  forms  of  armature. 


CHAPTER  XV. 

MOTORS  WITH  CIRCUITS  OF  DIFFERENT  RESISTANCE. 

As  lias  been  pointed  out  elsewhere,  the  lag;  or  retardation  of 
the  phases  of  an  alternating  current  is  directly  proportional  to 
the  self-induction  and  inversely  proportional  to  the  resistance  of 
the  circuit  through  which  the  current  flows.  Hence,  in  order 
to  secure  the  proper  differences  of  phase  between  the  two  motor- 
circuits,  it  is  desirable  to  make  the  self-induction  in  one  much 
higher  and  the  resistance  much  lower  than  the  self-induction  and 
resistance,  respectively,  in  the  other.  At  the  same  time  the 
magnetic  quantities  of  the  two  poles  or  sets  of  poles  which  the 
two  circuits  produce  should  be  approximately  equal.  These 
requirements  have  led  Mr.  Tesla  to  the  invention  of  a  motor 
having  the  following  general  characteristics :  The  coils  which 
are  included  in  that  energizing  circuit  which  is  to  have  the 
higher  self-induction  are  made  of  coarse  wire,  or  a  conductor  of 
relatively  low  resistance,  and  with  the  greatest  possible  length 
or  number  of  turns.  In  the  other  set  of  coils  a  comparatively 
few  turns  of  liner  wire  are  used,  or  a  wire  of  higher  resistance. 
Furthermore,  in  order  to  approximate  the  magnetic  quantities  of 
the  poles  excited  by  these  coils,  Mr.  Tesla  employs  in  the  self- 
induction  circuit  cores  much  longer  than  those  in  the  other  or 
resistance  circuit. 

Fig.  65  is  a  part  sectional  view  of  the  motor  at  right  angles  to 
the  shaft.  Fig.  66  is  a  diagram  of  the  tield  circuits. 

In  Fig.  66,  let  A  represent  the  coils  in  one  motor  circuit,  and  H 
those  in  the  other.  The  circuit  A  is  to  have  the  higher  self- 
induction.  There  are,  therefore,  used  a  long  length  or  a  large 
number  of  turns  of  coarse  wire  in  forming  the  coils  of  this  cir- 
cuit. For  the  circuit  B,  a  smaller  conductor  is  employed,  or  a 
conductor  of  a  higher  resistance  than  copper,  such  as  German 
silver  or  iron,  and  the  coils  are  wound  with  fewer  turns.  In  apply- 
ing these  coils  to  a  motor,  Mr.  Tesla  builds  up  a  field-magnet  of 
plates  c,  of  iron  and  steel,  secured  together  in  the  usual  manner 


80  INVENTIONS  OF  NIKOLA  TESLA. 

by  bolts  D.  Each  plate  is  formed  with  four  (more  or  less)  long 
cores  E,  around  which  is  a  space  to  receive  the  coil  and  an  equal 
number  of  short  projections  F  to  receive  the  coils  of  the  resistance- 
circuit.  The  plates  are  generally  annular  in  shape,  having  an 
open  space  in  the  centre  for  receiving  the  armature  G,  which  Mr. 
Tesla  prefers  to  wind  with  closed  coils.  An  alternating  current 
divided  between  the  two  circuits  is  retarded  as  to  its  phases  in 
the  circuit  A  to  a  mucli  greater  extent  than  in  the  circuit  B.  By 


FIG.  65. 


FIG. 


reason  of  the  relative  sizes  and  disposition  of  the  cores  and  coils 
the  magnetic  effect  of  the  poles  E  and  F  upon  the  armature  closely 
approximate. 

An  important  result  secured  by  the  construction  shown  here 
is  that  these  coils  which  are  designed  to  have  the  higher  self- 
induction  are  almost  completely  surrounded  by  iron,  and  that  the 
retardation  is  thus  very  materially  increased. 


CHAPTER  XVI. 

MOTOR  WITH  EQUAL  MAGNETIC  ENERGIES  IN  FIELD  AND 
ARMATURE. 

LET  it  be  assumed  that  the  energy  as  represented  in  the  magnet- 
ism in  the  field  of  a  given  rotating  field  motor  is  ninety  and 
thafe  of  the  armature  ten.  The  sum  of  these  quantities,  which 
represents  the  total  energy  expended  in  driving  the  motor,  is 
one  hundred;  but,  assuming  that  the  motor  be  so  constructed 
that  the  energy  in  the  field  is  represented  by  fifty,  and  that  in 
the  armature  by  fifty,  the  sum  is  still  one  hundred ;  but  while  in 
the  first  instance  the  product  is  nine  hundred,  in  the  second  it  is 


FIG.  67. 

two  thousand  five  hundred,  and  as  the  energy  developed  is  in 
proportion  to  these  products  it  is  clear  that  those  motors  are  the 
most  efficient — other  things  being  equal — in  which  the  magnetic 
energies  developed  in  the  armature  and  field  are  equal.  These 
results  Mr.  Tesla  obtains  by  using  the  same  amount  of  copper  or 
ampere  turns  in  both  elements  when  the  cores  of  both  are  equal, 
or  approximately  so,  and  the  same  current  energizes  both ;  or  in 
cases  where  the  currents  in  one  element  are  induced  to  those  of 
the  other  he  uses  in  the  induced  coils  an  excess  of  copper  over 
that  in  the  primary  element  or  conductor. 


S3  INVENTIONS  OF  NIKOLA  TESLA. 

The  conventional  figure  of  a  motor  here  introduced,  Fig.  H7, 
will  give  an  idea  of  the  solution  furnished  by  Mr.  Tesla  for  the 
specific  problem.  Referring  to  the  drawing,  A  is  the  field-mag- 
net, B  the  armature,  c  the  field  coils,  and  D  the  armature-coils  of 
the  motor. 

Generally  speaking,  if  the  mass  of  the  cores  of  armature  and 
field  be  equal,  the  amount  of  copper  or  ampere  turns  of  the 
energizing  coils  on  both  should  also  be  equal ;  but  these  condi- 
tions will  be  modified  in  different  forms  of  machine.  It  will  be 
understood  that  these  results  are  most  advantageous  when  exist- 
ing under  the  conditions  presented  where  the  motor  is  running 
with  its  normal  load,  a  point  to  be  well  borne  in  mind. 


CHAPTER   XVII. 

MOTORS  WITH  COINCIDING  MAXIMA  OF  MAGNETIC  EFFECT  IN 
ARMATURE  AND  FIELD. 

IN  THIS  forin  of  motor,  Mr.  Tesla's  object  is  to  design  and 
build  machines  wherein  the  maxima  of  the  magnetic  effects  of 
the  armature  and  field  will  more  nearly  coincide  than  in  some  of 
the  types  previously  under  consideration.  These  types  are :  First, 
motors  having  two  or  more  energizing  circuits  of  the  same  elec- 
trical character,  and  in  the  operation  of  which  the  currents  used 
differ  primarily  in  phase;  second,  motors  with  a  plurality  of 
energizing  circuits  of  different  electrical  character,  in  or  by 
means  of  which  the  difference  of  phase  is  produced  artificially, 
and,  third,  motors  with  a  plurality  of  energizing  circuits,  the 
currents  in  one  being  induced  from  currents  in  another.  Con- 
sidering the  structural  and  operative  conditions  of  any  one  of 
them — as,  for  example,  that  first  named — the  armature  which  is 
mounted  to  rotate  in  obedience  to  the  co-operative  influence  or 
action  of  the  energizing  circuits  has  coils  wound  upon  it  which 
are  closed  upon  themselves  and  in  which  currents  are  induced  by 
the  energizing-currents  with  the  object  and  result  of  energizing 
the  armature-core ;  but  under  any  such  conditions  as  must  exist 
in  these  motors,  it  is  obvious  that  a  certain  time  must  elapse 
between  the  manifestations  of  an  energizing  current  impulse  in 
the  field  coils,  and  the  corresponding  magnetic  state  or  phase  in 
the  armature  established  by  the  current  induced  thereby;  conse- 
quently a  given  magnetic  influence  or  effect  in  the  field  which  is 
the  direct  result  of  a  primary  current  impulse  will  have  become 
more  or  less  weakened  or  lost  before  the  corresponding  effect  in 
the  armature  indirectly  produced  has  reached  its  maximum.  This 
is  a  condition  unfavorable  to  efficient  working  in  certain  cases — as, 
for  instance,  when  the  progress  of  the  resultant  poles  or  points 
of  maximum  attraction  is  verj*  great,  or  when  a  very  high  num- 
ber of  alternations  is  employed— for  it  is  apparent  that  a  stronger 


84 


INVENTIONS  OF  NIKOLA  TESLA. 


tendency  to  rotation  will  be  maintained  if  the  maximum  mag- 
netic attractions  or  conditions  in  both  armature  and  field  coincide, 
the  energy  developed  by  a  motor  being  measured  by  the  product 
of  the  magnetic  quantities  of  the  armature  and  field. 

To  secure  this  coincidence  of  maximum  magnetic  effects,  Mr. 
Tesla  has  devised  various  means,  as  explained  below.  Fig.  68  is 
a  diagrammatic  illustration  of  a  Tesla  motor  system  in  which  the 
alternating  currents  proceed  from  independent  sources  and  differ 
primarily  in  phase. 

A  designates  the  field-magnet  or  magnetic  frame  of  the  motor; 


FIG.  68. 


FIG.  69. 


B  B,  oppositely  located  pole-pieces  adapted  to  receive  the  coils  of 
one  energizing  circuit ;  and  c  c,  similar  pole-pieces  for  the  coils 
of  the  other  energizing  circuit.  These  circuits  are  designated, 
respectively,  by  D  E,  the  conductor  B"  forming  a  common  return 
to  the  generator  G.  Between  these  poles  is  mounted  an  armature 
— for  example,  a  ring  or  annular  armature,  wound  with  a  series 
of  coils  F,  forming  a  closed  circuit  or  circuits.  The  action  or 
operation  of  a  motor  thus  constructed  is  now  well  understood. 
It  will  be  observed,  however,  that  the  magnetism  of  poles  B,  for 


POLYPHASK  CURRENTS.  85 

example,  established  by  a  current  impulse  in  the  coils  thereon, 
precedes  the  magnetic  effect  set  up  in  the  armature  by  the  in- 
duced current  in  coils  F.  Consequently  the  mutual  attraction 
between  the  armature  and  field-poles  is  considerably  reduced. 
The  same  conditions  will  be  found  to  exist  if,  instead  of  assuming 
the  poles  B  or  c  as  acting  independently,  we  regard  the  ideal  re- 
sultant of  both  acting  together,  which  is  the  real  condition.  To 
remedy  this,  the  motor  field  is  constructed  with  secondary  poles 
B'  c',  which  are  situated  between  the  others.  These  pole-pieces 
are  wound  with  coils  D'  E',  the  former  in  derivation  to  the  coils 
D,  the  latter  to  coils  E.  The  main  or  primary  coils  D  and  E  are 
wound  for  a  different  self-induction  from  that  of  the  coils*D'  and 
E',  the  relations  being  so  fixed  that  if  the  currents  in  D  and  E 
differ,  for  example,  by  a  quarter-phase,  the  currents  in  each 
secondary  coil,  as  D'  E',  will  differ  from  those  in  its  appropriate 
primary  D  or  E  by,  say,  forty-five  degrees,  or  one-eighth  of  a 
period. 

Now,  assuming  that  an  impulse  or  alternation  in  circuit  or 
branch  E  is  just  beginning,  while  in  the  branch  u  it  is  just  falling 
from  maximum,  the  conditions  are  those  of  a  quarter-phase 
difference.  The  ideal  resultant  of  the  attractive  forces  of  the  two 
sets  of  poles  B  c  therefore  may  be  considered  as  progressing  from 
poles  B  to  poles  c,  while  the  impulse  in  E  is  rising  to  maximum, 
and  that  in  D  is  falling  to  zero  or  minimum.  The  polarity  set  up 
in  the  armature,  however,  lags  behind  the  manifestations  of  field 
magnetism,  and  hence  the  maximum  points  of  attraction  in  arma- 
ture and  field,  instead  of  coinciding,  are  angularly  displaced. 
This  effect  is  counteracted  by  the  supplemental  poles  B'  c'.  The 
magnetic  phases  of  these  poles  succeed  those  of  poles  B  c  by  the 
same,  or  nearly  the  same,  period  of  time  as  elapses  between  the 
effect  of  the  poles  B  c  and  the  corresponding  induced  effect  in  the 
armature ;  hence  the  magnetic  conditions  of  poles  B'  c'  and  of 
the  armature  more  nearly  coincide  and  a  better  result  is  obtained. 
As  poles  B'  c'  act  in  conjunction  with  the  poles  in  the  armature 
established  by  poles  B  c,  so  in  turn  poles  c  B  act  similarly  with 
the  poles  set  up  by  B'  c',  respectively.  Under  such  conditions 
the  retardation  of  the  magnetic  effect  of  the  armature  and  that 
of  the  secondary  poles  will  bring  the  maximum  of  the  two  more 
nearly  into  coincidence  and  a  correspondingly  stronger  torque  or 
magnetic  attraction  secured. 

In  such  a  disposition  as  is  shown  in  Fig.  fiS  it  will  be  observed 


86 


INVENTIONS  OF  NIKOLA  TESLA. 


that  as  the  adjacent  pole-pieces  of  either  circuit  are  of  like  polar- 
ity they  will  have  a  certain  weakening  effect  upon  one  another. 
Mr.  Tesla  therefore  prefers  to  remove  the  secondary  poles  from 
the  direct  influence  of  the  others.  This  may  be  done  by  con- 
structing a  motor  with  two  independent  sets  of  fields,  and  with 
either  one  or  two  armatures  electrically  connected,  or  by  using 
two  armatures  and  one  field.  These  modifications  are  illustrated 
further  on. 

Fig.  69  is  a  diagrammatic  illustration  of  a  motor  and  system  in 
which  the  difference  of  phase  is  artificially  produced.  There  are 
two  coils  D  i)  in  one  branch  and  two  coils  E  E  in  another  branch 


FIG.  71. 


of  the  main  circuit  from  the  generator  o.  These  two  circuits  or 
branches  are  of  different  self-induction,  one,  as  D,  being  higher 
than  the  other.  This  is  graphically  indicated  by  making  coils  D 
much  larger  than  coils  E.  By  reason  of  the  difference  in  the 
electrical  character  of  the  two  circuits,  the  phases  of  current  in 
one  are  retarded  to  a  greater  extent  than  the  other.  Let  this 
difference  be  thirty  degrees.  A  motor  thus  constructed  will 
rotate  under  the  action  of  an  alternating  current ;  but  as  happens 
in  the  case  previously  described  the  corresponding  magnetic  ef- 
fects of  the  armature  and  field  do  not  coincide  owing  to  the  time 
that  elapses  between  a  given  magnetic  effect  in  the  armature  and 


POLYPHASE  CURRENTS  87 

the  condition  of  the  field  that  produces  it.  The  secondary  or 
supplemental  poles  B'  c'  are  therefore  availed  of.  There  being 
thirty  degrees  difference  of  phase  between  the  currents  in  coils 
D  E,  the  magnetic  effect  of  poles  B'  c'  should  correspond  to  that 
produced  by  a  current  differing  from  the  current  in  coils  D  or  K 
by  fifteen  degrees.  This  we  can  attain  by  winding  each  supple- 
mental pole  B'  c'  with  two  coils  H  H'.  The  coils  H  are  included 
in  a  derived  circuit  having  the  same  self-induction  as  circuit  D, 
and  coils  H'  in  a  circuit  having  the  same  self-induction  as  circuit 
E,  so  that  if  these  circuits  differ  by  thirty  degrees  the  magnetism 
of  poles  B'  c'  will  correspond  to  that  produced  by  a  current  dif- 
fering from  that  in  either  D  or  E  by  fifteen  degrees.  This  is  true 
in  all  other  cases.  For  example,  if  in  Fig.  68  the  coils  D'  E'  be 
replaced  by  the  coils  H  H'  included  in  the  derived  circuits,  the 
magnetism  of  the  poles  B'  c'  will  correspond  in  effect  or  phase, 
if  it  may  be  so  termed,  to  that  produced  by  a  current  differing 
from  that  in  either  circuit  D  or  E  by  forty-five  degrees,  or  one- 
eighth  of  a  period. 

This  invention  as  applied  to  a  derived  circuit  motor  is  illustra- 
ted in  Figs.  70  and  71.  The  former  is  an  end  view  of  the  motor 
with  the  armature  in  section  and  a  diagram  of  connections,  and 
Fig.  71  a  vertical  section  through  the  field.  These  figures  are 
also  drawn  to  show  one  of  the  dispositions  of  two  fields  that  may 
be  adopted  in  carrying  out  the  principle.  The  poles  B  B  c  c  are 
in  one  field,  the  remaining  poles  in  the  other.  The  former  are 
wound  with  primary  coils  i  j  and  secondary  coils  i'  j',  the  latter 
with  coils  K  L.  The  primary  coils  i  j  are  in  derived  circuits,  be- 
tween which,  by  reason  of  their  different  self-induction,  there  is 
a  difference  of  phase,  say,  of  thirty  degrees.  The  coils  i'  K  are 
in  circuit  with  one  another,  as  also  are  coils  j'  L,  and  there  should 
be  a  difference  of  phase  between  the  currents  in  coils  K  and  L  and 
their  corresponding  primaries  of,  say,  fifteen  degrees.  If  the 
poles  B  c  are  at  right  angles,  the  armature-coils  should  be  con- 
.nected  directly  across,  or  a  single  armature  core  wound  from  end 
to  end  may  be  used ;  but  if  the  poles  B  c  be  in  line  there  should 
be  an  angular  displacement  of  the  armature  coils,  as  will  be  well 
understood. 

The  operation  will  be  understood  from  the  foregoing.  The 
maximum  magnetic  condition  of  a  pair  of  poles,  as  B'  B',  coincides 
closely  with  the  maximum  effect  in  the  armature,  which  lags  be- 
hind the  corresponding  condition  in  poles  H  n. 


CHAPTER  XVIII. 

MOTOR  BASED  ON  THE  DIFFERENCE  OF  PHASE  IN  THE  MAGNETIZA- 
TION OF  THE  INNER  AND  OUTER  PARTS  OF  AN  IRON  CORE. 

IT  is  well  known  that  if  a  magnetic  core,  even  if  laminated  or 
subdivided,  be  wound  with  an  insulated  coil  and  a  current  of 
electricity  be  directed  through  the  coil,  the  magnetization  of  the 
entire  core  does  not  immediately  ensue,  the  magnetizing  effect 
not  being  exhibited  in  all  parts  simultaneously.  This  may  be  at- 
tributed to  the  fact  that  the  action  of  the  current  is  to  energize 
first  those  laminae  or  parts  of  the  core  nearest  the  surface  and 
adjacent  to  the  exciting-coil,  and  from  thence  the  action  pro- 
gresses toward  the  interior.  A  certain  interval  of  time  therefore 
elapses  between  the  manifestation  of  magnetism  in  the  external 
and  the  internal  sections  or  layers  of  the  core.  If  the  core  be 
thin  or  of  small  mass,  this  effect  may  be  inappreciable ;  but  in 
the  case  of  a  thick  core,  or  even  of  a  comparatively  thin  one,  if 
the  number  of  alternations  or  rate  of  change  of  the  current 
strength  be  very  great,  the  time  interval  occurring  between  the 
manifestations  of  magnetism  in  the  interior  of  the  core  and  in 
those  parts  adjacent  to  the  coil  is  more  marked.  In  the  con- 
struction of  such  apparatus  as  motors  which  are  designed  to  be 
run  by  alternating  or  equivalent  currents — such  as  pulsating  or 
undulating  currents  generally — Mr.  Tesla  found  it  desirable  and 
even  necessary  to  give  due  consideration  to  this  phenomenon  and 
to  make  special  provisions  in  order  to  obviate  its  consequences. 
With  the  specific  object  of  taking  advantage  of  this  action  or 
effect,  and  to  render  it  more  pronounced,  he  constructs  a  field 
magnet  in  which  the  parts  of  the  core  or  cores  that  exhibit  at 
different  intervals  of  time  the  magnetic  effect  imparted  to  them 
by  alternating  or  equivalent  currents  in  an  energizing  coil  or  coils, 
are  so  placed  with  relation  to  a  rotating  armature  as  to  exert 
thereon  their  attractive  effect  successively  in  the  order  of  their 
magnetization.  By  this  means  he  secures  a  result  similar  to  that 
which  he  had  previously  attained  in  other  forms  or  types  of  mo- 


POLYPHASE  CURRENTS.  89 

tor  in  which  by  means  of  one  or  more  alternating  currents  he 
lias  produced  the  rotation  or  progression  of  the  magnetic  poles. 

This  new  mode  of  operation  will  now  be  described.  Fig.  72 
is  a  side  elevation  of  such  motor.  Fig.  73  is  a  side  elevation  of 
a  more  practicable  and  efficient  embodiment  of  the  invention. 
Fig.  74  is  a  central  vertical  section  of  the  same  in  the  plane  of 
the  axis  of  rotation. 

Referring  to  Fig.  72,  let  x  represent  a  large  iron  core,  which 
may  be  composed  of  a  number  of  sheets  or  laminae  of  soft  iron 
or  steel.  Surrounding  this  core  is  a  coil  Y,  which  is  connected 
with  a  source  E  of  rapidly  varying  currents.  Let  us  consider  now 


FIGS.  72  and  73. 

the  magnetic  conditions  existing  in  this  core  at  any  point,  as  5, 
at  or  near  the  centre,  and  any  other  point,  as  #,  nearer  the  sur- 
face. When  a  current  impulse  is  started  in  the  magnetizing  coil 
Y,  the  section  or  part  at  <z,  being  close  to  the  coil,  is  immediately 
energized,  while  the  section  or  part  at  J,  which,  to  use  a  conveni- 
ent expression,  is  "  protected "  by  the  intervening  sections  or 
layers  between  a  and  J,  does  not  at  once  exhibit  its  magnetism. 
However,  as  the  magnetization  of  a  increases,  5  becomes  also 
affected,  reaching  finally  its  maximum  strength  some  time  later 
than  a.  Upon  the  weakening  of  the  current  the  magnetization 
of  a  first  diminishes,  while  J  still  exhibits  its  maximum  strength ; 


90  INVENTIONS  OF  NIKOLA   TESLA. 

but  the  continued  weakening  of  a  is  attended  by  a  subsequent 
weakening  of  b.  Assuming  the  current  to  be  an  alternating  one, 
a  will  now  be  reversed,  while  b  still  continues  of  the  first  imparted 
polarity.  This  action  continues  the  magnetic  condition  of  &,  fol- 
lowing that  of  a  in  the  manner  above  described.  If  an  armature 
— for  instance,  a  simple  disc  F,  mounted  to  rotate  freely  on  an 
axis — be  brought  into  proximity  to  the  core,  a  movement  of  rota- 
tion will  be  imparted  to  the  disc,  the  .direction  depending  upon 
its  position  relatively  to  the  core,  the  tendency  being  to  turn  the 
portion  of  the  disc  nearest  to  the  core  from  a  to  £>,  as  indicated 
in  Fig.  72. 

This  action  or  principle  of  operation  has  been  embodied  in  a 
practicable  form  of  motor,  which  is  illustrated  in  Fig.  73.     Let  A 


FIG.  74. 

in  that  figure  represent  a  circular  frame  of  iron,  from  diametric- 
ally opposite  points  of  the  interior  of  which  the  cores  project. 
Each  core  is  composed  of  three  main  parts  B,  B  and  c,  and  they 
are  similarly  formed  with  a  straight  portion  or  body  <?,  around 
which  the  energizing  coil  is  wound,  a  curved  arm  or  extension  e, 
and  an  inwardly  projecting  pole  or  end  d.  Each  core  is  made  up 
of  two  parts  B  B,  with  their  polar  extensions  reaching  in  one 
direction,  and  a  part  c  between  the  other  two,  and  with  its  polar 
extension  reaching  in  the  opposite  direction.  In  order  to  lessen 
in  the  cores  the  circulation  of  currents  induced  therein,  the  several 
sections  are  insulated  from  one  another  in  the  manner  usually 


POLYPHASE  CURRENTS.  91 

followed  in  such  cases.  These  cores  are  wound  with  coils  D,  which 
are  connected  in  the  same  circuit,  either  in  parallel  or  series,  arid 
supplied  with  an  alternating  or  a  pulsating  current,  preferably 
the  former,  by  a  generator  K,  represented  diagrammatically.  Be- 
tween the  cores  or  their  polar  extensions  is  mounted  a  cylindrical 
or  similar  armature  F,  wound  with  magnetizing  coils  G,  closed 
upon  themselves. 

The  operation  of  this  motor  is  as  follows :  When  a  current 
impulse  or  alternation  is  directed  through  the  coils  D,  the  sections 
B  B  of  the  cores,  being  on  the  surface  and  in  close  proximity  to 
the  coils,  are  immediately  energized.  The  sections  c,  on  the  other 
hand,  are  protected  from  the  magnetizing  influence  of  the  coil 
by  the  interposed  layers  of  iron  B  B.  As  the  magnetism  of  B  B 
increases,  however,  the  sections  c  are  also  energized ;  but  they 
do  not  attain  their  maximum  strength  until  a  certain  time  subse- 
quent to  the  exhibition  by  the  sections  B  B  of  their  maximum. 
Upon  the  weakening  of  the  current  the  magnetic  strength  of  B  B 
first  diminishes,  while  the  sections  c  have  still  their  maximum 
strength ;  but  as  B  B  continue  to  weaken  the  interior  sections  are 
similarly  weakened.  B  B  may  then  begin  to  exhibit  an  opposite 
polarity,  which  is  followed  later  by  a  similar  change  on  c,  and 
this  action  continues.  B  B  and  c  may  therefore  be  considered  as 
separate  field-magnets,  being  extended  so  as  to  act  on  the  arma- 
ture in  the  most  efficient  positions,  and  the  effect  is  similar  to 
that  in  the  other  forms  of  Tesla  motor — viz.,  a  rotation  or  pro- 
gression of  the  maximum  points  of  the  field  of  force.  Any 
armature — such,  for  instance,  as  a  disc — mounted  in  this  field 
would  rotate  from  the  pole  first  to  exhibit  its  magnetism  to  that 
which  exhibits  it  later. 

It  is  evident  that  the  principle  here  described  may  be  carried 
out  in  conjunction  with  other  means  for  securing  a  more  favor- 
able or  efficient  action  of  the  motor.  For  example,  the  polar 
extensions  of  the  sections  c  may  be  wound  or  surrounded  by 
closed  coils.  The  effect  of  these  coils  will  be  to  still  more 
effectively  retard  the  magnetization  of  the  polar  extensions  of  c. 


CHAPTER  XIX. 

ANOTHER  TYPE  OF  TESLA  INDUCTION  MOTOR. 

IT  WILL  have  been  gathered  by  all  who  are  interested  in  the 
advance  of  the  electrical  arts,  and  who  follow  carefully,  step  by 
step,  the  work  of  pioneers,  that  Mr.  Tesla  ha£  been  foremost  to 
utilize  inductive  effects  in  permanently  closed  circuits,  in  the 
operation  of  alternating  motors.  In  this  chapter  one  simple  type 
of  such  a  motor  is  described  and  illustrated,  which  will  serve  as 
an  exemplification  of  the  principle. 

Let  it  be  assumed  that  an  ordinary  alternating  current  genera- 
tor is  connected  up  in  a  circuit  of  practically  no  self-induction, 
such,  for  example,  as  a  circuit  containing  incandescent  lamps 
only.  On  the  operation  of  the  machine,  alternating  currents  will 
be  developed  in  the  circuit,  and  the  phases  of  these  currents  will 
theoretically  coincide  with  the  phases  of  the  impressed  electro- 
motive force.  Such  currents  may  be  regarded  and  designated  as 
the  "unretarded  currents." 

It  will  be  understood,  of  course,  that  in  practice  there  is  al- 
ways more  or  less  self-induction  in  the  circuit,  which  modifies  to 
a  corresponding  extent  these  conditions ;  but  for  convenience 
this  may  be  disregarded  in  the  consideration  of  the  principle  of 
operation,  since  the  same  laws  apply.  Assume  next  that  a  path 
of  currents  be  formed  across  any  two  points  of  the  above  cir- 
cuit, consisting,  for  example,  of  the  primary  of  an  induction  de- 
vice. The  phases  of  the  currents  passing  through  the  primary, 
owing  to  the  self-induction  of  the  same,  will  not  coincide  with 
the  phases  of  the  impressed  electromotive  force,  but  will  lag- 
behind,  such  lag  being  directly  proportional  to  the  self-induction 
and  inversely  proportional  to  the  resistance  of  the  said  coil. 
The  insertion  of  this  coil  will  also  cause  a  lagging  or  retardation 
of  the  currents  traversing  and  delivered  by  the  generator  behind 
the  impressed  electromotive  force,  such  lag  being  the  mean  or 
resultant  of  the  lag  of  the  current  through  the  primary  alone  and 
of  the  "  unretarded  current "  in  the  entire  working  circuit.  Next 


POL  YPHAXE  CURRENTS. 


88 


consider  the  conditions  imposed  by  the  association  in  inductive 
relation  with  the  primary  coil,  of  a  secondary  coil.  The  current 
generated  in  the  secondary  coil  will  react  upon  the  primary  cur- 
rent, modifying  the  retardation  of  the  same,  according  to  the 
amount  of  self-induction  and  resistance  in  the  secondary  circuit. 
If  the  secondary  circuit  has  but  little  self-induction — as,  for  in- 
stance, when  it  contains  incandescent  lamps  only — it  will  in- 
crease the  actual  difference  of  phase  between  its  own  and  the 
primary  current,  first,  by  diminishing  the  lag  between  the  pri- 
mary current  and  the  impressed  electromotive  force,  and,  sec- 
ond, by  its  own  lag  or  retardation  behind  the  impressed  electro- 
motive force.  On  the  other  hand,  if  the  secondary  circuit  have 
a  high  self-induction,  its  lag  behind  the  current  in  the  primary  is 


FIG.  7.-). 

directly  increased,  while  it  will  be  still  further  increased  if  the 
primary  have  a  very  low  self-induction.  The  better  results  are 
obtained  when  the  primary  has  a  low  self-induction. 

Fig.  75  is  a  diagram  of  a  Tesla  motor  embodying  this  princi- 
ple. Fig.  76  is  a  similar  diagram  of  a  modification  of  the  same. 
In  Fig.  75  let  A  designate  the  field-magnet  of  a  motor  which,  as 
in  all  these  motors,  is  built  up  of  sections  or  plates.  B  c  are  po- 
lar projections  upon  which  the  coils  are  wound.  Upon  one  pair 
of  these  poles,  as  c,  are  wound  primary  coils  i>,  which  are  di- 
rectly connected  to  the  circuit  of  an  alternating  current  genera- 
tor a.  On  the  same  poles  are  also  wound  secondary  coils  r, 
either  side  by  side  or  over  or  under  the  primary  coils,  and  these 
are  connected  with  other  coils  E,  which  surround  the  poles  B  B. 


94  INVENTIONS  OF  NIKOLA  TESLA. 

The  currents  in  both  primary  and  secondary  coils  in  such  a  mo- 
tor will  be  retarded  or  will  lag  behind  the  impressed  electro- 
motive force  ;  but  to  secure  a  proper  difference  in  phase  between 
the  primary  and  secondary  currents  themselves,  Mr.  Tesla  in- 
creases the  resistance  of  the  circuit  of  the  secondary  and  reduces 
as  much  as  practicable  its  self-induction.  This  is  done  by  using 
for  the  secondary  circuit,  particularly  in  the  coils  E,  wire  of  com- 
paratively small  diameter  and  having  but  few  turns  around  the 
cores;  or  by  using  some  conductor  of  higher  specific  resistance, 
such  as  German  silver ;  or  by  introducing  at  some  point  in  the 
secondary  circuit  an  artificial  resistance  K.  Thus  the  self-induc- 
tion of  the  secondary  is  kept  down  and  its  resistance  increased, 
with  the  result  of  decreasing  the  lag  between  the  impressed 
electro-motive  force  and  the  current  in  the  primary  coils  and  in- 
creasing the  difference  of  phase  between  the  primary  and  secon- 
dary currents. 

In  the  disposition  shown  in  Fig.  76,  the  lag  in  the  secondary 
is  increased  by  increasing  the  self-induction  of  that  circuit,  while 
the  increasing  tendency  of  the  primary  to  lag  is  counteracted  by 
inserting  therein  a  dead  resistance.  The  primary  coils  D  in  this 
case  have  a  low  self-induction  and  high  resistance,  while  the  coils 
E  F,  included  in  the  secondary  circuit,  have  a  high  self-induction 
and  low  resistance.  This  may  be  done  by  the  proper  winding  of 
the  coils ;  or  in  the  circuit  including  the  secondary  coils  E  F,  we 
may  introducb  a  self-induction  coil  s,  while  in  the  primary  cir- 
cuit from  the  generator  o  and  including  coils  D,  there  may  be  in 
serted  a  dead  resistance  R.  By  this  means  the  difference  of 
phase  between  the  primary  and  secondary  is  increased.  It  is  evi- 
dent that  both  means  of  increasing  the  difference  of  phase — 
namely,  by  the  special  winding  as  well  as  by  the  supplemental  or 
external  inductive  and  dead  resistance — may  be  employed  con- 
jointly. 

In  the  operation  of  this  motor  the  current  impulses  in  the  pri- 
mary coils  induce  currents  in  the  secondary  coils,  and  by  the  con- 
joint action  of  the  two  the  points  of  greatest  magnetic  attraction 
are  shifted  or  rotated. 

In  practice  it  is  found  desirable  to  wind  the  armature  with 
closed  coils  in  which  currents  are  induced  by  the  action  thereon 
of  the  primaries. 


CHAPTER  XX. 

COMBINATIONS  OF  SYNCHRONIZING    MOTOR  AND  TORQUE   MOTOR. 

IN  THE  preceding  descriptions  relative  to  synchronizing  motors 
and  methods  of  operating  them,  reference  has  been  made  to  the 
plan  adopted  by  Mr.  Tesla,  which  consists  broadly  in  winding  or 
arranging  the  motor  in  such  manner  that  by  means  of  suitable 
switches  it  could  be  started  as  a  multiple-circuit  motor,  or  one 
operating  by  a  progression  of  its  magnetic  poles,  and  then,  when 
up  to  speed,  or  nearly  so,  converted  into  an  ordinary  synchroniz- 
ing motor,  or  one  in  which  the  magnetic  poles  were  simply  alter- 
nated. In  some  cases,  as  when  a  large  motor  is  used  and  when 
the  number  of  alternations  is  very  high,  there  is  more  or  less 
difficulty  in  bringing  the  motor  to  speed  as  a  double  or  multiple- 
circuit  motor,  for  the  plan  of  construction  which  renders  the 
motor  best  adapted  to  run  as  a  synchronizing  motor  impairs  its 
efficiency  as  a  torque  or  double-circuit  motor  under  the  assumed 
conditions  on  the  start.  This  will  be  readily  understood,  for  in  a 
large  synchronizing  motor  the  length  of  the  magnetic  circuit  of 
the  polar  projections,  and  their  mass,  are  so  great  that  apparently 
considerable  time  is  required  for  magnetization  and  demagnetiza- 
tion. Hence  with  a  current  of  a  very  high  number  of  alternations 
the  motor  may  not  respond  properly.  To  avoid  this  objection 
and  to  start  up  a  synchronizing  motor  in  which  these  conditions 
obtain,  Mr.  Tesla  has  combined  two  motors,  one  a  synchronizing 
motor,  the  other  a  multiple-circuit  or  torque  motor,  and  by  the 
latter  he  brings  the  first-named  up  to  speed,  and  then  either 
throws  the  whole  current  into  the  synchronizing  motor  or  operates 
jointly  both  of  the  motors. 

This  invention  involves  several  novel  and  useful  features.  It 
will  be  observed,  in  the  first  place,  that  both  motors  are  run, 
without  commutators  of  any  kind,  and,  secondly,  that  the  speed 
of  the  torque  motor  may  be  higher  than  that  of  the  synchroniz- 
ing motor,  as  will  be  the  case  when  it  contains  a  fewer  number  of 
poles  or  sets  of  poles,  so  that  the  motor  will  be  more  readily  and 


96  INVENTIONS  OF  NIKOLA  TESLA. 

easily  brought  up  to  speed.  Thirdly,  the  synchronizing  motor 
may  be  constructed  so  as  to  have  a  much  more  pronounced  ten- 
dency to  synchronism  without  lessening  the  facility  with  which 
it  is  started. 

Fig.  77  is  a  part  sectional  view  of  the  two  motors ;  Fig.  78  an 
end  view  of  the  synchronizing  motor ;  Fig.  79  an  end  view  and 
part  section  of  the  torque  or  double-circuit  motor;  Fig.  80  a 
diagram  of  the  circuit  connections  employed ;  and  Figs.  81,  82, 
83,  84  and  85  are  diagrams  of  modified  dispositions  of  the  two 
motors. 

Inasmuch  as  neither  motor  is  doing  any  work  while  the  current 
is  acting  upon  the  other,  the  two  armatures  are  rigidly  connected, 
both  being  mounted  upon  the  same  shaft  A,  the  field-magnets  B 
of  the  synchronizing  and  c  of  the  torque  motor  being  secured  to 


the  same  base  D.  The  preferably  larger  synchronizing  motor  has 
polar  projections  on  its  armature,  which  rotate  in  very  close  prox- 
imity to  the  poles  of  the  field,  and  in  other  respects  it  conforms 
to  the  conditions  that  are  necessary  to  secure  synchronous  action. 
The  pole-pieces  of  the  armature  are,  however,  wound  with  closed 
coils  E,  as  this  obviates  the  employment  of  sliding  contacts.  The 
smaller  or  torque  motor,  on  the  other  hand,  has,  preferably,  a 
cylindrical  armature  F,  without  polar  projections  and  wound  with 
closed  coils  G.  The  field-coils  of  the  torque  motor  are  connected 
up  in  two  series  H  and  i,  and  the  alternating  current  from  the 
generator  is  directed  through  or  divided  between  these  two  cir- 
cuits in  any  manner  to  produce  a  progression  of  the  poles  or 
points  of  maximum  magnetic  effect.  This  result  is  secured  by 
connecting  the  two  motor-circuits  in  derivation  witli  the  circuit 


POLYPHASE  CURRENTS. 


from  the  generator,  inserting  in  one  motor  circuit  a  dead  resist- 
ance and  in  the  other  a  self-induction  coil,  by  which  means  a 
difference  in  phase  between  the  two  divisions  of  the  current  is 
secured.  If  both  motors  have  the  same  number  of  field  poles, 
the  torque  motor  for  a  given  number  of  alternations  will  tend  to 
run  at  double  the  speed  of  the  other,  for,  assuming  the  connec- 
tions to  be  such  as  to  give  the  best  results,  its  poles  are  divided 
into  two  series  and  the  number  of  poles  is  virtually  reduced  one- 
half,  which  being  acted  upon  by  the  same  number  of  alternations 
tend  to  rotate  the  armature  at  twice  the  speed.  By  this  means 
the  main  armature  is  more  easily  brought  to  or  above  the  required 
speed.  -When  the  speed  necessary  for  synchronism  is  imparted 
to  the  main  motor,  the  current  is  shifted  from  the  torque  motor 
into  the  other. 

A  convenient  arrangement  for  carrying  out  this  invention  is 


FIG.  78. 


FIG.  79. 


shown  in  Fig.  80,  in  which  j  .1  are  the  field  coils  of  the  syn- 
chronizing, and  H  i  the  field  coils  of  the  torque  motor.  L  L'  are 
the  conductors  of  the  main  line.  One  end  of,  say,  coils  H  is  con- 
nected to  wire  L  through  a  self-induction  coil  M.  One  end  of  the 
other  set  of  coils  i  is  connected  to  the  same  wire  through  a  dead 
resistance  N.  The  opposite  ends  of  these  two  circuits  are  con- 
nected to  the  contact  m  of  a  switch,  the  handle  or  lever  of  which 
is  in  connection  with  the  line-wire  L',  One  end  of  the  field  cir- 
cuit of  the  synchronizing  motor  is  connected  to  the  wire  L.  The 
other  terminates  in  the  switch-contact  n.  From  the  diagram  it 
will  be  readily  seen  that  if  the  lever  p  be  turned  upon  contact  m, 
the  torque  motor  will  start  by  reason  of  the  difference  of  phase 
between  the  currents  in  its  two  energizing  circuits.  Then  when 
the  desired  speed  is  attained,  if  the  lever  p  be  shifted  upon  con- 


98  INVENTIONS  OF  NIKOLA  TESLA. 

tact  ;/  the  entire  current  will  pass  through  the  field  coils  of  the 
synchronizing  motor  and  the  other  will  be  doing  no  work. 

The  torque  motor  may  be  constructed  and  operated  in  various 
ways,  many  of  which  have  already  been  touched  upon.  It  is  not 
necessary  that  one  motor  be  cut  out  of  circuit  while  the  other  is 
in,  for  both  may  be  acted  upon  by  current  at  the  same  time,  and 
Mr.  Tesla  has  devised  various  dispositions  or  arrangements  of  the 
two  motors  for  accomplishing  this.  Some  of  these  arrangements 
are  illustrated  in  Figs.  81  to  85. 

Referring  to  Fig.  81,  let  T  designate  the  torque  or  multiple 
circuit  motor  and  s  the  synchronizing  motor,  L  i,'  being  the  line- 
wires  from  a  source  of  alternating  current.  The  two  circuits  of 
the  torque  motor  of  different  degrees  of  self-induction,  and  de- 
signated by  N  M,  are  connected  in  derivation  to  the  wire  L.  They 
are  then  joined  and  connected  to  the  energizing  circuit  of  the 


FIG. 


synchronizing  motor,  the  opposite  terminal  of  which  is  connected 
to  wire  L'.  The  two  motors  are  thus  in  series.  To  start  them 
Mr.  Tesla  short-circuits  the  synchronizing  motor  by  a  switch  P', 
throwing  the  whole  current  through  the  torque  motor.  Then 
when  the  desired  speed  is  reached  the  switch  p'  is  opened,  so 
that  the  current  passes  through  both  motors.  In  such  an  arrange- 
ment as  this  it  is  obviously  desirable  for  economical  and  other 
reasons  that  a  proper  relation  between  the  speeds  of  the  two 
motors  should  be  observed. 

In  Fig.  82  another  disposition  is  illustrated,  s  is  the  synchron- 
izing motor  and  T  the  torque  motor,  the  circuits  of  both  being  in 
parallel,  w  is  a  circuit  also  in  derivation  to  the  motor  circuits 
and  containing  a  switch  P".  s'  is  a  switch  in  the  synchronizing 
motor  circuit.  On  the  start,  the  switch  s'  is  opened,  cutting  out 
the  motor  s.  Then  P"  is  opened,  throwing  the  entire  current 


POLYPHASE  CURRENTS. 


through  the  motor  T,  giving  it  a  very  strong  torque.     When  the 
desired  speed  is  reached,  switch  s'  is  closed  and  the  current  divides 


FIGS.  81,  82,  83,  84  and  85. 

between  both  motors.     By  means  of  switch  p"  both  motors  may 
be  cut  out. 


100  INVENTIONS  OF  NIKOLA  TE8LA. 

In  Fig.  83  the  arrangement  is  substantially  the  same,  except 
that  a  switch  T'  is  placed  in  the  circuit  which  includes  the  two  cir- 
cuits of  the  torque  motor.  Fig.  84  shows  the  two  motors  in 
series,  with  a  shunt  around  both  containing  a  switch  s  T.  There 
is  also  a  shunt  around  the  synchronizing  motor  s,  with  a  switch 
p'.  In  Fig.  85  the  same  disposition  is  shown ;  but  each  motor  is 
provided  witli  a  shunt,  in  which  are  switches  P'  and  T*,  as  shown. 


CHAPTER   XXL 

MOTOR  WITH  A  CONDENSER  IN  THE  ARMATURE  CIRCUIT. 

WE  NOW  come  to  a  new  class  of  motors  in  which  resort  is  had 
to  condensers  for  the  purpose  of  developing  the  required  differ- 
ence of  phase  and  neutralizing  the  effects  of  self-induction.  Mr. 
Tesla  early  began  to  apply  the  condenser  to  alternating  appara- 
tus, in  just  howr  many  ways  can  only  be  learned  from  a  perusal 
of  other  portions  of  this  volume,  especially  those  dealing  with 
his  high  frequency  work. 

Certain  laws  govern  the  action  or  effects  produced  by  a  con- 
denser when  connected  to  an  electric  circuit  through  which  an 
alternating  or  in  general  an  undulating  current  is  made  to  pass. 
Some  of  the  most  important  of  such  effects  are  as  follows :  First, 
if  the  terminals  or  plates  of  a  condenser  be  connected  with  two 
points  of  a  circuit,  the  potentials  of  which  are  made  to  rise  and 
fall  in  rapid  succession,  the  condenser  allows  the  passage,  or  more 
strictly  speaking,  the  transference  of  a  current,  although  its 
plates  or  armatures  may  be  so  carefully  insulated  as  to  prevent 
almost  completely  the  passage  of  a  current  of  unvarying  strength 
or  direction  and  of  moderate  electromotive  force.  Second,  if  a 
circuit,  the  terminals  of  which  are  connected  with  the  plates  of 
the  condenser,  possess  a  certain  self-induction,  the  condenser  will 
overcome  or  counteract  to  a  greater  or  less  degree,  dependent 
upon  well-understood  conditions,  the  effects  of  such  self-induc- 
tion. Third,  if  two  points  of  a  closed  or  complete  circuit 
through  wThich  a  rapidly  rising  and  falling  current  flows  be 
shunted  or  bridged  by  a  condenser,  a  variation  in  the  strength  of 
the  currents  in  the  branches  and  also  a  difference  of  phase  of  the 
currents  therein  is  produced.  These  effects  Mr.  Tesla  has  utilized 
and  applied  in  a  variety  of  ways  in  the  construction  and  operation 
of  his  motors,  such  as  by  producing  a  difference  in  phase  in  the 
two  energizing  circuits  of  an  alternating  current  motor  by  con- 
necting the  two  circuits  in  derivation  and  connecting  up  a  con- 
denser in  series  in  one  of  the  circuits.  A  further  development, 


102 


INVENTIONS  OF  NIKOLA  TESLA. 


however,  possesses  certain  novel  features  of  practical  value  and  in- 
volves a  knowledge  of  facts  less  generally  understood.  It  comprises 
the  use  of  a  condenser  or  condensers  in  connection  with  the  induced 
or  armature  circuit  of  a  motor  and  certain  details  of  the  con- 


.  87. 


FIG.  90. 


struction  of  such  motors.  In  an  alternating  current  motor  of  the 
type  particularly  referred  to  above,  or  in  any  other  which  has 
an  armature  coil  or  circuit  closed  upon  itself,  the  latter  repre- 
sents not  only  an  inductive  resistance,  but  one  which  is  period- 


POLYPHASE  VUKRENT8.  103 

ically  varying  in  value,  both  of  which  facts  complicate  and  render 
difficult  the  attainment  of  the  conditions  best  suited  to  the  most 
efficient  working  conditions ;  in  other  words,  they  require,  first, 
that  for  a  given  inductive  effect  upon  the  armature  there  should 
be  the  greatest  possible  current  through  the  armature  or  induced 
coils,  and,  second,  that  there  should  always  exist  between  the 
currents  in  the  energizing  and  the  induced  circuits  a  given  rela- 
tion of  phase.  Hence  whatever  tends  to  decrease  the  self-induc- 
tion and  increase  the  current  in  the  induced  circuits  will,  other 
things  being  equal,  increase  the  output  arid  efficiency  of  the  mo- 
tor, and  the  same  will  be  true  of  causes  that  operate  to  maintain 
the  mutual  attractive  effect  between  the  field  magnets  and  arma- 
ture at  its  maximum.  Mr.  Tesla  secures  these  results  by  con- 
necting with  the  induced  circuit  or  circuits  a  condenser,  in  the 
manner  described  below,  and  he  also,  with  this  purpose  in  view, 
constructs  the  motor  in  a  special  manner. 

Referring  to  the  drawings,  Fig.  86,  is  a  view,  mainly  dia- 
grammatic, of  an  alternating  current  motor,  in  which  the  present 
principle  is  applied.  Fig.  87  is  a  central  section,  in  line  with 
the  shaft,  of  a  special  form  of  armature  core.  Fig.  88  is  a  simi- 
lar section  of  a  modification  of  the  same.  Fig.  89  is  one  of  the 
sections  of  the  core  detached.  Fig.  90  is  a  diagram  showing  a 
modified  disposition  of  the  armature  or  induced  circuits. 

The  general  plan  of  the  invention  is  illustrated  iji  Fig.  86. 
A  A  in  this  figure  represent  the  the  frame  and  field  magnets  of 
an  alternating  current  motor,  the  poles  or  projections  of  which 
are  wound  with  coils  B  and  c,  forming  independent  energizing 
circuits  connected  either  to  the  same  or  to  independent  sources 
of  alternating  currents,  so  that  the  currents  flowing  through  the 
circuits,  respectively,  will  have  a  difference  of  phase.  Within 
the  influence  of  this  field  is  an  armature  core  D,  wound  with  coils 
E.  In  motors  of  this  description  heretofore  these  coils  have  been 
closed  upon  themselves,  or  connected  in  a  closed  series;  but  in 
the  present  case  each  coil  or  the  connected  series  of  coils  termi- 
nates in  the  opposite  plates  of  a  condenser  F.  For  this  purpose 
the  ends  of  the  series  of  coils  are  brought  out  through  the  shaft 
to  collecting  rings  G,  which  are  connected  to  the  condenser  by 
contact  brushes  H  and  suitable  conductors,  the  condenser  being 
independent  of  the  machine.  The  armature  coils  are  wound  or 
connected  in  such  manner  that  adjacent  coils  produce  opposite 
poles. 


104  INVENTIONS  OF  NIKOLA  TK8LA. 

The  action,  of  this  motor  and  the  effect  of  the  plan  followed 
in  its  construction  are  as  follows :  The  motor  being  started  in 
operation  and  the  coils  of  the  field  magnets  being  traversed  by 
alternating  currents,  currents  are  induced  in  the  armature  coils 
by  one  set  of  field  coils,  as  B,  and  the  poles  thus  established  are 
acted  upon  by  the  other  set,  as  c.  The  armature  coils,  however, 
have  necessarily  a  high  self-induction,  which  opposes  the  flow  of 
the  currents  thus  set  up.  The  condenser  F  not  only  permits  the 
passage  or  transference  of  these  currents,  but  also  counteracts 
the  effects  of  self-induction,  and  by  a  proper  adjustment  of  the 
capacity  of  the  condenser,  the  self-induction  of  the  coils,  and  the 
periods  of  the  currents,  the  condenser  may  be  made  to  overcome 
entirely  the  effect  of  self-induction. 

It  is  preferable  on  account  of  the  undesirability  of  using  sliding 
contacts  of  any  kind,  to  associate  the  condenser  with  the  armature 
directly,  or  make  it  a  part  of  the  armature.  In  some  cases  Mr. 
Tesla  builds  up  the  armature  of  annular  plates  K  K,  held  by  bolts 
L  between  heads  M,  which  are  secured  to  the  driving  shaft,  and 
in  the  hollow  space  thus  formed  he  places  a  condenser  F,  gener- 
ally by  winding  the  two  insulated  plates  spirally  around  the 
shaft.  In  other  cases  he  utilizes  the  plates  of  the  core  itself 
as  the  plates  of  the  condenser.  For  example,  in  Figs.  88  and  89, 
N  is  the  driving  shaft,  M  M  are  the  heads  of  the  armature-core, 
and  K  K'  the  iron  plates  of  which  the  core  is  built  up.  These 
plates  are  insulated  from  the  shaft  and  from  one  another,  and  are 
held  together  by  rods  or  bolts  L.  The  bolts  pass  through  a  large 
hole  in  one  plate  and  a  small  hole  in  the  one  next  adjacent,  and 
so  on,  connecting  electrically  all  of  plates  K,  as  one  armature  of  a 
condenser,  and  all  of  plates  K'  as  the  other. 

To  either  of  the  condensers  above  described  the  armature  coils 
may  be  connected,  as  explained  by  reference  to  Fig.  86. 

In  motors  in  which  the  armature  coils  are  closed  upon  them- 
selves— as,  for  example,  in  any  form  of  alternating  current  motor 
in  which  one  armature  coil  or  set  of  coils  is  in  the  position  of 
maximum  induction  with  respect  to  the  field  coils  or  poles,  while 
the  other  is  in  the  position  of  minimum  induction — the  coils  are 
best  connected  in  one  series,  and  two  points  of  the  circuit 
thus  formed  are  bridged  by  a  condenser.  This  is  illustrated  in 
Fig.  90,  in  which  E  represents  one  set  of  armature  coils  and  E' 
the  other.  Their  points  of  uniou  are  joined  through  a  con- 
denser F.  It  will  be  observed  that  in  this  disposition  the  self- 


POLYPHASE  CURRENTS.  105 

induction  of  the  two  branches  E  and  E'  varies  with  their  position 
relatively  to  the  field  magnet,  and  that  each  branch  is  alternately 
the  predominating  source  of  the  induced  current.  Hence  the 
effect  of  the  condenser  F  is  twofold.  First,  it  increases  the  cur- 
rent in  each  of  the  branches  alternately,  and,  secondly,  it  alters 
the  phase  of  the  currents  in  the  branches,  this  being  the  well- 
known  effect  which  results  from  such  a  disposition  of  a  con- 
denser with  a  circuit,  as  above  described.  This  effect  is  favorable 
to  the  proper  working  of  the  motor,  because  it  increases  the  flow 
of  current  in  the  armature  circuits  due  to  a  given  inductive 
effect,  and  also  because  it  brings  more  nearly  into  coincidence 
the  maximum  magnetic  effects  of  the  coacting  field  and  armature 
poles. 

It  will  be  understood,  of  course,  that  the  causes  that  contri- 
bute to  the  efficiency  of  condensers  when  applied  to  such  uses  as 
the  above  must  be  given  due  consideration  in  determining  the 
practicability  and  efficiency  of  the  motors.  Chief  among  these 
is,  as  is  well  known,  the  periodicity  of  the  current,  and  hence  the 
improvements  described  are  mgre  particularly  adapted  to  systems 
in  which  a  very  high  rate  of  alternation  or  change  is  main- 
tained. 

Although  this  invention  has  been  illustrated  in  connection 
with  a  special  form  of  motor,  it  will  be  understood  that  it  is 
equally  applicable  to  any  other  alternating  current  motor  in 
which  there  is  a  closed  armature  coil  wherein  the  currents  are 
induced  by  the  action  of  the  field,  and  the  feature  of  utilizing 
the  plates  or  sections  of  a  magnetic  core  for  forming  the  con- 
denser is  applicable,  generally,  to  other  kinds  of  alternating  cur- 
rent apparatus. 


CHAPTEK  XXII. 

MOTOR  WITH  CONDENSER  IN  ONE  OF  THE  FIELD  CIRCUITS. 

IF  THE  field  or  energizing  circuits  of  a  rotary  phase  motor  be 
both  derived  from  the  same  source  of  alternating  currents  and  a 
condenser  of  proper  capacity  be  included  in  one  of  the  same,  ap- 
proximately, the  desired  difference  of  phase  may  be  obtained  be- 
tween the  currents  flowing  directly  from  the  source  and  those 
flowing  through  the  condenser ;  but  the  great  size  and  expense 
of  condensers  for  this  purpose  that  would  meet  the  requirements 
of  the  ordinary  systems  of  comparatively  low  potential  are  par- 
ticularly prohibitory  to  their  employment. 

Another,  now  well-known,  method  or  plan  of  securing  a  differ- 
ence of  phase  between  the  energizing  currents  of  motors  of  this 
kind  is  to  induce  by  the  currents  in  one  circuit  those  in  the  other 
circuit  or  circuits ;  but  as  no  means  had  been  proposed  that 
would  secure  in  this  way  between  the  phases  of  the  primary  or 
inducing  and  the  secondary  or  induced  currents  that  difference — 
theoretically  ninety  degrees— that  is  best  adapted  for  practical 
and  economical  working,  Mr.  Tesla  devised  a  means  which  ren- 
ders practicable  both  the  above  described  plans  or  methods,  and 
by  which  he  is  enabled  to  obtain  an  economical  and  efficient  al- 
ternating current  motor.  His  invention  consists  in  placing  a 
condenser  in  the  secondary  or  induced  circuit  of  the  motor  above 
described  and  raising  the  potential  of  the  secondary  currents  to 
such  a  degree  that  the  capacity  of  the  condenser,  which  is  in 
part  dependent  on  the  potential,  need  be  quite  small.  The  value 
of  this  condenser  is  determined  in  a  well-understood  manner  with 
reference  to  the  self-induction  and  other  conditions  of  the  circuit, 
so  as  to  cause  the  currents  which  pass  through  it  to  differ  from 
the  primary  currents  by  a  quarter  phase. 

Fig.  91  illustrates  the  invention  as  embodied  in  a  motor 
in  which  the  inductive  relation  of  the  primary  and  secondary 
circuits  is  secured  by  winding  them  inside  the  motor  partly 
upon  the  same  cores ;  but  the  invention  applies,  generally,  to 


POL  YPHA&K  d 


107 


other  forms  of  motor  in  which  one  of  the  energizing  currents  is 
induced  in  any  way  from  the  other. 

Let  A  B  represent  the  poles  of  an  alternating  current  motor,  of 
which  c  is  the  armature  wound  with  coils  D,  closed  upon  them- 
selves, as  is  now  the  general  practice  in  motors  of  this  kind.  The 
poles  A,  which  alternate  with  poles  B,  are  wound  with  coils  of 
ordinary  or  coarse  wire  E  in  such  direction  as  to  make  them  of 
alternate  north  and  south  polarity,  as  indicated  in  the  diagram 
by  the  characters  N  s.  Over  these  coils,  or  in  other  inductive  re- 
lation to  the  same,  are  wound  long  fine-wire  coils  F  F,  and  in  the 


FIG.  91. 

same  direction  throughout  as  the  coils  E.  These  coils  are  secon- 
daries, in  which  currents  of  very  high  potential  are  induced.  All 
the  coils  E  in  one  series  are  connected,  and  all  the  secondaries  F 
in  another. 

On  the  intermediate  poles  B  are  wound  line-wire  energizing 
coils  G,  which  are  connected  in  series  with  one  another,  and  also 
with  the  series  of  secondary  coils  F,  the  direction  of  winding  be- 
.ing  such  that  a  current-impulse  induced  from  the  primary  coils 
K  imparts  the  same  magnetism  to  the  poles  B  as  that  produced 


108  INVENTIONS  OF  NIKOLA  TESLA. 

in  poles  A  by  the  primary  impulse.     Tins  condition  is  indicated 
by  the  characters  N'  s'. 

In  the  circuit  formed  by  the  two  sets  of  coils  F  and  G  is  intro- 
duced a  condenser  H  ;  otherwise  this  circuit  is  closed  upon 
itself,  while  the  free  ends  of  the  circuit  of  coils  E  are  connected 
to  a  source  of  alternating  currents.  As  the  condenser  capacity 
which  is  needed  in  any  particular  motor  of  this  kind  is  depend- 
ent upon  the  rate  of  alternation  or  the  potential,  or  both,  its  size 
or  cost,  as  before  explained,  may  be  brought  within  economical 
limits  for  use  with  the  ordinary  circuits  if  the  potential  of  the 
secondary  circuit  in  the  motor  be  sufficiently  high.  By  giving 
to  the  condenser  proper  values,  any  desired  difference  of  phase 
between  the  primary  and  secondary  energizing  circuits  may  be 
obtained. 


CHAPTER  XXIII. 

TESLA  POLYPHASE  TRANSFORMER. 

APPLYING  the  polyphase  principle  to  the  construction  of  trans- 
formers as  well  to  the  motors  already  noticed,  Mr.  Tesla  has  in- 
vented some  very  interesting  forms,  which  he  considers  free 
from  the  defects  of  earlier  and,  at  present,  more  familiar  forms. 
In  these  transformers  he  provides  a  series  of  inducing  coils  and 
corresponding  induced  coils,  which  are  generally  wound  upon  a 
core  closed  upon  itself,  usually  a  ring  of  laminated  iron. 

The  two  sets  of  coils  are  wound  side  by  side  or  superposed  or 
otherwise  placed  in  well-known  ways  to  bring  them  into  the  most 
effective  relations  to  one  another  and  to  the  core.  The  inducing 
or  primary  coils  wound  on  the  core  are  divided  into  pairs  or  sets 
by  the  proper  electrical  connections,  so  that  while  the  coils  of 
one  pair  or  set  co-operate  in  fixing  the  magnetic  poles  of  the 
core  at  two  given  diametrically  opposite  points,  the  coils  of  the 
other  pair  or  set — assuming,  for  sake  of  illustration,  that  there 
are  but  two — tend  to  fix  the  poles  ninety  degrees  from  such 
points.  With  this  induction  device  is  used  an  alternating  current 
generator  with  coils  or  sets  of  coils  to  correspond  with  those  of 
the  converter,  and  the  corresponding  coils  of  the  generator  and 
converter  are  then  connected  up  in  independent  circuits.  It  re- 
sults from  this  that  the  different  electrical  phases  in  the  genera- 
tor are  attended  by  corresponding  magnetic  changes  in  the  con- 
verter; or,  in  other  words,  that  as  the  generator  coils  revolve, 
the  points  of  greatest  magnetic  intensity  in  the  converter  will  be 
progressively  shifted  or  whirled  around. 

Fig.  92  is  a  diagrammatic  illustration  of  the  converter  and  the 
electrical  connections  of  the  same.  Fig.  93  is  a  horizontal  cen- 
tral cross-section  of  Fig.  92.  Fig.  94  is  a  diagram  of  the  circuits 
of  the  entire  system,  the  generator  being  shown  in  section. 

Mr.  Tesla  uses  a  core,  A,  which  is  closed  upon  itself — that  is  to 
say,  of  an  annular  cylindrical  or  equivalent  form — and  as  the 
efficiency  of  the  apparatus  is  largely  increased  by  the  subdivision 


no 


INVENTIONS  0V  NIKOLA    TKKLA. 


of  tliis  core,  he  makes  it  of  thin  strips,  plates,  or  wires  .of  soft 
iron  electrically  insulated  as  far  as  practicable.  Upon  this  core 
are  wound,  say,  four  coils,  BBS'  B',  used  as  primary  coils,  and  'for 
which  long  lengths  of  comparatively  fine  wire  are  employed. 
Over  these  coils  are  then  wound  shorter  coils  of  coarser  wire,  c  c 
c'  c',  to  constitute  the  induced  or  secondary  coils.  The  construc- 
tion of  this  or  any  equivalent  form  of  converter  may  be  carried 
further,  as  above  pointed  out,  by  inclosing  these  coils  with  iron 
— as,  for  example,  by  winding  over  the  coils  layers  of  insulated 
iron  wire. 

The  device  is  provided  with  suitable  binding  posts,  to  which 


FIGS.  92  and  93. 

the  ends  of  the  coils  are  led.  The  diametrically  opposite  coils 
B  R  and  B'  B'  are  connected,  respectively,  in  series,  and  the  four 
terminals  are  connected  to  the  binding  posts.  The  induced 
coils  are  connected  together  in  any  desired  manner.  For  ex- 
ample, as  shown  in  Fig.  94,  c  c  may  be  connected  in  multiple 
arc  when  a  quantity  current  is  desired— as  for  running  a  group 
of  incandescent  lamps— while  c'  c'  may  be  independently  con- 
nected in  series  in  a  circuit  including  arc  lamps  or  the  like.  The 
generator  in  this  system  will  be  adapted  to  the  converter  in  the 


POLYPHASE  CURRENTS. 


Ill 


manner  illustrated.  For  example,  in  the  present  case  there  are 
employed  a  pair  of  ordinary  permanent  or  electro-magnets,  E  E, 
between  which  is  mounted  a  cylindrical  armature  on  a  shaft,  F, 
and  wound  with  two  coils,  G  G'.  The  terminals  of  these  coils  are 
connected,  respectively,  to  four  insulated  contact  or  collecting 
rings,  H  H  H'  H',  and  the  four  line  circuit  wires  L  connect  the 
brushes  K,  bearing  on  these  rings,  to  the  converter  in  the  order 
shown.  Noting  the  results  of  this  combination,  it  will  be  ob- 
served that  at  a  given  point  of  time  the  coil  G  is  in  its  neutral 
position  and  is  generating  little  or  no  current,  while  the  other 
coil,  G',  is  in  a  position  where  it  exerts  its  maximum  effect. 
Assuming  coil  G  to  be  connected  in  circuit  with  coils  B  B  of  the 
converter,  and  coil  G'  with  coils  B'  B',  it  is  evident  that  the  poles 


FIG.  94. 


of  the  ring  A  will  be  determined  by  coils  B'  B'  alone ;  but  as  the 
armature  of  the  generator  revolves,  coil  G  develops  more  current 
and  coil  G'  less,  until  G  reaches  its  maximum  and  G'  its  neutral 
position.  The  obvious  result  will  be  to  shift  the  poles  of  the 
ring  A  through  one-quarter  of  its  periphery.  The  movement  of 
the  coils  through  the  next  quarter  of  a  turn — during  which  coil 
(/  enters  a  tield  of  opposite  polarity  and  generates  a  current  of 
opposite  direction  and  increasing  strength,  while  coil  G,  in  passing 
from  its  maximum  to  its  neutral  position  generates  a  current  of 
decreasing  strength  and  same  direction  as  before — causes  a  further 
shifting  of  the  poles  through  the  second  quarter  of  the  ring. 
The  second  half -re  volution  will  obviously  be  a  repetition  of  the 
same  action.  By  the  shifting  of  the  poles  of  the  ring  A,  a  power- 


112  INVENTIONS  OF  NIKOLA  TE8LA. 

ful  dynamic  inductive  effect  on  the  coils  c  c'  is  produced.  Be- 
sides the  currents  generated  in  the  secondary  coils  by  dynamo- 
magnetic  induction,  other  currents  will  be  set  up  in  the  same 
coils  in  consequence  of  many  variations  in  the  intensity  of  the 
poles  in  the  ring  A.  This  should  be  avoided  by  maintaining  the 
intensity  of  the  poles  constant,  to  accomplish  which  care  should 
be  taken  in  designing  and  proportioning  the  generator  and  in 
distributing  the  coils  in  the  ring  A,  and  balancing  their  effect. 
When  this  is  done,  the  currents  are  produced  by  dynamo-mag- 
netic induction  only,  the  same  result  being  obtained  as  though 
the  poles  were  shifted  by  a  commutator  with  an  infinite  number 
of  segments. 

The  modifications  which  are  applicable  to  other  forms  of  con- 
verter are  in  many  respects  applicable  to  this,  such  as  those  per- 
taining more  particularly  to  the  form  of  the  core,  the  relative 
lengths  and  resistances  of  the  primary  and  secondary  coils,  and 
the  arrangements  for  running  or  operating  the  same. 


CHAPTEK  XXIV. 

A   CONSTANT  CURRENT   TRANSFORMER   WITH   MAGNETIC    SHIELD 
BETWEEN  COILS  OF  PRIMARY  AND  SECONDARY. 

MR.  TESLA  has  applied  his  principle  of  magnetic  shielding  of 
parts  to  the  construction  also  of  transformers,  the  shield  being 
interposed  between  the  primary  and  secondary  coils.  In  trans- 
formers of  the  ordinary  type  it  will  be  found  that  the  wave  of 
electromotive  force  of  the  secondary  very  nearly  coincides  with 
that  of  the  primary,  being,  however,  in  opposite  sign.  At  the  same 
time  the  currents,  both  primary  and  secondary,  lag  behind  their 
respective  electromotive  forces ;  but  as  this  lag  is  practically  or 
nearly  the  same  in  the  case  of  each  it  follows  that  the  maximum 
and  minimum  of  the  primary  and  secondary  currents  will  nearly 
coincide,  but  differ  in  sign  or  direction,  provided  the  secondary 
be  not  loaded  or  if  it  contain  devices  having  the  property  of 
self-induction.  On  the  other  hand,  the  lag  of  the  primary 
behind  the  impressed  electromotive  force  may  be  diminished  by 
loading  the  secondary  with  a  non-inductive  or  dead  resistance — 
such  as  incandescent  lamps — whereby  the  time  interval  between 
the  maximum  or  minimum  periods  of  the  primary  and  secondary 
currents  is  increased.  This  time  interval,  however,  is  limited, 
and  the  results  obtained  by  phase  difference  in  the  operation  of 
such  devices  as  the  Tesla  alternating  current  motors  can  only  be 
approximately  realized  by  such  means  of  producing  or  securing 
this  difference,  as  above  indicated,  for  it  is  desirable  in  such  cases 
that  there  should  exist  between  the  primary  and  secondary  cur- 
rents, or  those  which,  however  produced,  pass  through  the  two 
circuits  of  the  motor,  a  difference  of  phase  of  ninety  degrees; 
or,  in  other  words,  the  current  in  one  circuit  should  be  a  maxi- 
mum when  that  in  the  other  circuit  is  a  minimum.  To  attain 
to  this  condition  more  perfectly,  an  increased  retardation  of  the 
secondary  current  is  secured  in  the  following  manner:  Instead 
of  bringing  the  primary  and  secondary  coils  or  circuits  of  a 
transformer  into  the  closest  possible  relations,  as  has  hitherto 


114 


INVENTIONS  OF  NIKOLA  TESLA. 


been  done,  Mr.  Tesla  protects  in  a  measure  the  secondary  from 
the  inductive  action  or  effect  of  the  primary  by  surrounding 
either  the  primary  or  the  secondary  with  a  comparatively  thin 
magnetic  shield  or  screen.  Under  these  modified  conditions, 
as  long  as  the  primary  current  has  a  small  value,  the  shield 
protects  the  secondary;  but  as  soon  as  the  primary  current 
has  reached  a  certain  strength,  which  is  arbitrarily  determined, 
the  protecting  magnetic  shield  becomes  saturated  and  the  induc- 
tive action  upon  the  secondary  begins.  It  results,  therefore,  that 
the  secondary  current  begins  to  How  at  a  certain  fraction  of  a 
period  later  than  it  would  without  the  interposed  shield,  and 
since  this  retardation  may  be  obtained  without  necessarily  retard- 
ing the  primary  current  also,  an  additional  lag  is  secured,  and 
the  time  interval  between  the  maximum  or  minimum  periods  of 
the  primary  and  secondary  currents  is  increased.  Such  a  trans- 


FIG.  95. 

former  may,  by  properly  proportioning  its  several  elements  and 
determining  the  proper  relations  between  the  primary  and 
secondary  windings,  the  thickness  of  the  magnetic  shield,  and 
other  conditions,  be  constructed  to  yield  a  constant  current  at  all 
loads. 

Fig.  95  is  a  cross-section  of  a  transformer  embodying  this  im- 
provement. Fig.  96  is  a  similar  view  of  a  modified  form  of 
transformer,  showing  diagrammatically  the  manner  of  using  the 
same. 

A  A  is  the  main  core  of  the  transformer,  composed  of  a  ring 
of  soft  annealed  and  insulated  or  oxidized  iron  wire.  Upon  this 
core  is  wound  the  secondary  circuit  or  coil  B  B.  This  latter  is 
then  covered  with  a  layer  or  layers  of  annealed  and  insulated 
iron  wires  c  c,  wound  in  a  direction  at  right  angles  to  the  secondary 


POLYPHASE  CURRENTS. 


115 


coil.  Over  the  whole  is  then  wound  the  primary  coil  or  wire  D  D. 
From  the  nature  of  this  construction  it  will  be  obvious  that 
as  long  as  the  shield  formed  by  the  wires  c  is  below  magnetic 
saturation  the  secondary  coil  or  circuit  is  effectually  protected  or 
shielded  from  the  inductive  influence  of  the  primary,  although 
on  open  circuit  it  may  exhibit  some  electromotive  force.  When 
the  strength  of  the  primary  reaches  a  certain  value,  the  shield  c, 
becoming  saturated,  ceases  to  protect  the  secondary  from  induc- 
tive action,  and  current  is  in  consequence  developed  therein. 
For  similar  reasons,  when  the  primary  current  weakens,  the 
weakening  of  the  secondary  is  retarded  to  the  same  or  approxi- 
mately the  same  extent. 

The  specific  construction  of  the  transformer  is  largely  imma- 


FIG.  90. 

terial.  In  Fig.  90,  for  example,  the  core  A  is  built  up  of  thin 
insulated  iron  plates  or  discs.  The  primary  circuit  D  is  wound 
next  the  core  A.  Over  this  is  applied  the  shield  c,  which  in  this 
case  is  made  up  of  thin  strips  or  plates  of  iron  properly  insulated 
and  surrounding  the  primary,  forming  a  closed  magnetic  circuit. 
The  secondary  B  is  wound  over  the  shield  c.  In  Fig.  06,  also, 
K  is  a  source  of  alternating  or  rapidly  changing  currents. 
The  primary  of  the  transformer  is  connected  with  the  circuit  of 
the  generator.  F  is  a  two-circuit  alternating  current  motor,  one 
of  the  circuits  being  connected  with  the  main  circuit  from  the 
source  E,  and  the  other  being  supplied  with  currents  from  the 
secondarv  of  the  transformer. 


PART  II. 


THE  TESLA  EFFECTS  WITH  HIGH  FREQUENCY 
AND  HIGH  POTENTIAL  CURRENTS. 


CHAPTER  XXV. 

INTRODUCTION. — THE  SCOPE  OF  THE  TESLA  LECTURES. 

BEFORE  proceeding  to  study  the  three  Tesla  lectures  here 
presented,  the  reader  may  find  it  of  some  assistance  to  have  his 
attention  directed  to  the  main  points  of  interest  and  significance 
therein.  The  lirst  of  these  lectures  was  delivered  in  New  York, 
at  Columbia  College,  before  the  American  Institute  of  Electrical 
Engineers,  May  20, 1891.  The  urgent  desire  expressed  immedi- 
ately from  all  parts  of  Europe  for  an  opportunity  to  witness  the 
brilliant  and  unusual  experiments  with  which  the  lecture  was 
accompanied,  induced  Mr.  Tesla  to  go  to  England  early  in  1892, 
when  he  appeared  before  the  Institution  of  Electrical  Engineers, 
and  a  day  later,  by  special  request,  before  the  Royal  Institution. 
His  reception  was  of  the  most  enthusiastic  and  flattering  nature  on 
both  occasions.  He  then  went,  by  invitation,  to  France,  and  re- 
peated his  novel  demonstrations  before  the  Societe  Internationale 
des  Electriciens,  and  the  Societe  Frangaise  de  Physique.  Mr.  Tesla 
returned  to  America  in  the  fall  of  1892,  and  in  February,  1893,  de- 
livered his  third  lecture  before  the  Franklin  Institute  of  Philadel- 
phia, in  fulfilment  of  a  long  standing  promise  to  Prof.  Houston. 
The  following  week,  at  the  request  of  President  James  I.  Ayer, 
of  the  National  Electric  Light  Association,  the  same  lecture  was 
re-delivered  in  St.  Louis.  It  had  been  intended  to  limit  the  in- 
vitations to  members,  but  the  appeals  from  residents  in  the  city 
were  so  numerous  and  pressing  that  it  became  necessary  to  secure 
a  very  large  hall.  Hence  it  came  about  that  the  lecture  was 
listened  to  by  an  audience  of  over  5,000  people,  and  was  in  some 
parts  of  a  more  popular  nature  than  either  of  its  predecessors. 
Despite  this  concession  to  the  need  of  the  hour  and  occasion,  Mr. 
Tesla  did  not  hesitate  to  show  many  new  and  brilliant  experi- 
ments, and  to  advance  the  frontier  of  discovery  far  beyond  any 
point  he  had  theretofore  marked  publicly. 

We  may  now  proceed  to  a  running  review  of  the  lectures  them- 
selves. The  ground  covered  by  them  is  so  vast  that  only  the 


120  INVENTIONS  OF  NIKOLA  TKSLA. 

leading  ideas  and  experiments  can  here  be  touched  upon ;  besides, 
it  is  preferable  that  the  lectures  should  be  carefully  gone  over  for 
their  own  sake,  it  being  more  than  likely  that  each  student  will 
discover  a  new  beauty  or  stimulus  in  them.  Taking  up  the 
course  of  reasoning  followed  by  Mr.  Tesla  in  his  first  lecture,  it 
will  be  noted  that  he  started  out  with  the  recognition  of  the  fact, 
which  he  has  now  experimentally  demonstrated,  that  for  the  pro- 
duction of  light  waves,  primarily,  electrostatic  effects  must  be 
_^Jjrought  into  play,  and  continuedjrtudy  has  led  him  tpjtheLOpinion 
that_all  electrical  and  magnetic  effects  may  be  referred  to  ek-c- 
trostajtic  _irLolecular  _  forces.  This  opinion  finds  a  singular  con- 
firmation in  one  of  the  most  striking  experiments  which  he 
describes,  namely,  the  production  of  a  veritable  flame  by  the 
agitation  of  electrostatically  charged  molecules.  It  is  of  the 
highest  interest  to  observe  that  this  result  points  out  a  way  of 
obtaining  a  flame  which  consumes  no  material  and  in  which  no 
chemical  action  whatever  takes  place.  It  also  throws  a  light  on 
the  nature  of  the  ordinary  flame,  which  Mr.  Tesla  believes  to  be 
due  to  electrostatic  molecular  actions,  which,  if  true,  would  lead 
directly  to  the  idea  that  even  chemical  affinities  might  be  electro- 
static in  their  nature  and  that,  as  has  already  been  suggested, 
molecular  forces  in  general  may  be  referable  to  one  and  the  same 
cause.  This  singular  phenomenon  accounts  in  a  plausible  man- 
ner for  the  unexplained  fact  that  buildings  are  frequently  set  on 
fire  during  thunder  storms  with'out  having  been  at  all  struck  by 
-\v  lightning.  It  may  also  explain  the  total  disappearance  of  ships 
at  sea. 

One  of  the  striking  proofs  of  the  correctness  of  the  ideas  ad- 
vanced by  Mr.  Tesla  is  the  fact  that,  notwithstanding  the  employ- 
ment of  the  most  powerful  electromagnetic  inductive  effects,  but 
.feeble  luminosity  is  obtainable,  and  this  only  in  close  proximity 
to  the  source  of  disturbance;  whereas,  when  the  electrostatic- 
effects  are  intensified,  the  same  initial  energy  suffices  to  excite 
luminosity  at  considerable  distances  from  the  source.  That  there 
are  only  electrostatic  effects  active  seems  to  be  clearly  proved  by 
Mr.  Tesla's  experiments  with  an  induction  coil  operated  with 
alternating  currents  of  very  high  frequency.  He  shows  how 
tubes  may  be  made  to  glow  brilliantly  at  considerable  distances 
from  any  object  when  placed  in  a  powerful,  rapidly  alternating, 
electrostatic  field,  and  he  describes  many  interesting  phenomena 
observed  in  such  a  field.  His  experiments  open  up  the  possibility 


man  FHKQUENVY  AND  HIGH  POTENTIAL  CVKRKNT*.    121 

of  lighting  an  apartment  by  simply  creating  in  it  sucli  an  electro- 
static field,  and  this,  in  a  certain  way,  would  appear  to  be  the 
ideal  method  of  lighting  a  room,  as  it  would  allow  the  illuminat- 
ing device  to  be  freely  moved  about.  The  power  with  which 
these  exhausted  tubes,  devoid  of  any  electrodes,  light  up  is  cer- 
tainly remarkable. 

That  the  principle  propounded  by  Mr.  Tesla  is  a  broad  one  is 
evident  from  the  many  ways  in  which  it  may  be  practically  ap- 
plied. We  need  only  refer  to  the  variety  of  the  devices  shown 
or  described,  all  of  which  are  novel  in  character  and  will,  with- 
out doubt,  lead  to  further  important  results  at  the  hands  of  Mr. 
Tesla  and  other  investigators.  The  experiment,  for  instance,  of 
lighting  up  a  single  filament  or  block  of  refractory  material  with 
a  single  wire,  is  in  itself  sufficient  to  give  Mr.  Tesla's  work  the 
stamp  of  originality,  and  the  numerous  other  experiments  and 
effects  which  may  be  varied  at  will,  are  equally  new  and  interest- 
ing. Thus,  the  incandescent  filament  spinning  in  an  unex- 
hausted globe,  the  well-known  Crookes  experiment  on  open  cir- 
cuit, and  the  many  others  suggested,  will  not  fail  to  interest  the 
reader.  Mr.  Tesla  has  made  an  exhaustive  study  of  the  various 
forms  of  the  discharge  presented  by  an  induction  coil  when  op- 
erated with  these  rapidly  alternating  currents,  starting  from  the 
thread-like  discharge  and  passing  through  various  stages  to  the 
true  electric  flame. 

A  point  of  great  importance  in  the  introduction  of  high  ten- 
sion alternating  current  which  Mr.  Tesla  brings  out  is  the  neces- 
sity of  carefully  avoiding  all  gaseous  matter  in  the  high  tension 
apparatus.  He  shows  that,  at  least  with  very  rapidly  alternating 
currents  of  high  potential,  the  discharge  may  work  through  al- 
most any  practicable  thickness  of  the  best  insulators,  if  air  is 
present.  In  such  cases  the  air  included  within  the  apparatus  is 
violently  agitated  and  by  molecular  bombardment  the  parts  may 
be  so  greatly  heated  as  to  cause  a  rupture  of  the.  insulation. 
The  practical  outcome  of  this  is,  that,  whereas  with  steady  cur- 
rents, any  kind  of  insulation  may  be  used,  with  rapidly  alternat- 
ing currents  oils  will  probably  be  the  best  to  employ,  a  fact 
which  has  been  observed,  but  not  until  now  satisfactorily  ex- 
plained. The  recognition  of  the  above  fact  is  of  special  impor- 
tance in  the  construction  of  the  costly  commercial  induction  coils 
which  are  often  rendered  useless  in  an  unaccountable  manner. 
The  truth  of  these  views  of  Mr.  Tesla  is  made  evident  by  the  in- 


122  INVENTIONS  OF  NIKOLA  TESLA. 

.  teresting  experiments  illustrative  of  the  behavior  of  the  air  be- 
tween charged  surfaces,  the  luminous  streams  formed  by  the 
charged  molecules  appearing  even  when  great  thicknesses  of  tin- 
best  insulators  are  interposed  between  the  charged  surfaces. 
These  luminous  streams  afford  in  themselves  a  very  interesting 
study  for  the  experimenter.  With  these  rapidly  alternating  cur- 
rents they  become  far  more  powerful  and  produce  beautiful  light 
effects  when  they  issue  from  a  wire,  pinwheel  or  other  object  at- 
tached to  a  terminal  of  the  coil ;  and  it  is  interesting  to  note  that 
they  issue  from  a  ball  almost  as  freely  as  from  a  point,  when  the 
frequency  is  very  high. 

From  these  experiments  we  also  obtain  a  better  idea  of  the 
importance  of  taking  into  account  the  capacity  and  self-induction 
in  the  apparatus  employed  and  the  possibilities  offered  by  the 
use  of  condensers  in  conjunction  with  alternate  currents,  the  em- 
ployment of  currents  of  high  frequency,  among  other  things, 
making  it  possible  to  reduce  the  condenser  to  practicable  dirnen- 
(sions.  Another  point  of  interest  and  practical  bearing  is  the 
fact,  proved  by  Mr.  Tesla,  that  for  alternate  currents,  especially 
those  of  high  frequency,  insulators  are  required  possessing  a 
small  specific  inductive  capacity,  which  at  the  same  time  have  a 
high  insulating  power. 

Mr.  Tesla  also  makes  interesting  and  valuable  suggestion  in  re- 
gard to  the  economical  utilization  of  iron  in  machines  and  trans- 
formers. He  shows  how,  by  maintaining  by  continuous  magnet- 
ization a  flow  of  lines  through  the  iron,  the  latter  may  be  kept 
near  its  maximum  permeability  and  a  higher  output  and  economy 
may  be  secured  in  such  apparatus.  This  principle  may  prove  of 
considerable  commercial  importance  in  the  development  of  alter- 
nating systems.  Mr.  Tesla's  suggestion  that  the  same  result  can 
be  secured  by  heating  the  iron  by  hysteresis  and  eddy  currents, 
and  increasing  the  permeability  in  this  manner,  while  it  may  ap- 
pear less  practical,  nevertheless  opens  another  direction  for  inves- 
tigation and  improvement. 

The  demonstration  of  the  fact  that  with  alternating  currents 

of  high  frequency,  sufficient  energy  may  be  transmitted  under 

practicable  conditions  through  the  glass  of  an  incandescent  lamp 

by  electrostatic  or  electromagnetic  induction  may  lead  to  a  de- 

^-parture  in  the  construction  of  such  devices.     Another  important 

i  I  experimental  result  achieved  is  the  operation  of  lamps,  and  even 

\  1  .motors,  with  the  discharges  of  condensers,  this  method  affording 


HIGH  FREQUENCY  AND  HIGH  POTENTIAL  CURRENTS.      123 

a  means  of  converting  direct  or  alternating  currents.  In  this 
connection  Mr.  Tesla  advocates  the  perfecting  of  apparatus  capa- 
ble of  generating  electricity  of  high  tension  from  heat  energy, 
believing  this  to  be  a  better  way  of  obtaining  electrical  energy 
for  practical  purposes,  particularly  for  the  production  of  light. 

While  many  were  probably  prepared  to  encounter  curious 
phenomena  of  impedance  in  the  use  of  a  condenser  discharged 
disruptively,  the  experiments  shown  were  extremely  interesting 
on  account  of  their  paradoxical  character.  The  burning  of  an 
incandescent  lamp  at  any  candle  power  when  connected  across  a 
heavy  metal  bar,  the  existence  of  nodes  on  the  bar  and  the  possi- 
bility of  exploring  the  bar  by  means  of  an  ordinary  Garde w 
voltmeter,  are  all  peculiar  developments,  but  perhaps  the  most 
interesting  observation  is  the  phenomenon  of  impedance  observed 
in  the  lamp  with  a  straight  filament,  which  remains  dark  while 
the  bulb  glows. 

Mr.  Tesla's  manner  of  operating  an  induction  coil  by  means  of 
the  disruptive  discharge,  and  thus  obtaining  enormous  differences 
of  potential  from  comparatively  small  and  inexpensive  coils,  will 
be  appreciated  by  experimenters  and  will  find  valuable  applica- 
tion in  laboratories.  Indeed,  his  many  suggestions  and  hints  in 
regard  to  the  construction  and  use  of  apparatus  in  these  investi- 
gations will  be  highly  valued  and  will  aid  materially  in  future- 
research. 

The  London  lecture  was  delivered   twice.     In  its  first  form, 
before   the   Institution  of  Electrical  Engineers,  it  was  in  some 
respects  an  amplification  of  several  j>oints  not  specially  enlarged 
upon  in  the  JSTew  York  lecture,  but  brought  forward  many  addi- 
tional  discoveries   and   new   investigations.      Its    repetition,    in-.""-] 
another  form,  at  the  Royal  Institution,  was  due  to  Prof.  Dewar, 
who  with  Lord  Ray leigh,  manifested  a  most  lively  interest  in  Mr.    'j 
Tesla's  work,  and  whose  kindness  illustrated  once  more  the  strong     V  } 
English  love  of  scientific  truth  and  appreciation  of  its  votaries.      } 
As  an  indefatigable  experimenter,  Mr.  Tesla  was  certainly  no-^ 
where  more  at  home  than  in  the  haunts  of  Faraday,  and  as  the    / 
guest  of    Faraday's   successor.     This  Royal   Institution   lecture    W 
summed  up  the  leading  points  of  Mr.  Tesla's  work,  in  the  high    / 
potential,  high  frequency  field,  and  we  may  here  avail  ourselves  J 
of  so  valuable  a  summarization,  in  a  simple  form,  of  a  subject  by 
no  means  easv  of  comprehension   until   it  has  been  thoroughly 
studied. 


124  INVENTIONS  OF  NIKOLA   TKSLA. 

In  these  London  lectures,  among  the  many  notable  points  made 
was  first,  the  difficulty  of  constructing  the  alternators  to  obtain, 
the  very  high  frequencies  needed.  To  obtain  the  high  fre- 
quencies it  was  necessary  to  provide  several  hundred  polar  pro- 
jections, which  were  necessarily  small  and  offered  many  draw- 
backs, and  this  the  more  as  exceedingly  high  peripheral  speeds 
had  to  be  resorted  to.  In  some  of  the  first  machines  both  arma- 
ture and  field  had  polar  projections.  These  machines  produced 
a  curious  noise,  especially  when  the  armature  was  started  from 
the  state  of  rest,  the  field  being  charged.  The  most  efficient 
machine  was  found  to  be  one  with  a  drum  armature,  the  iron 
body  of  which  consisted  of  very  thin  wire  annealed  with  special 
care.  It  was,  of  course,  desirable  to  avoid  the  employment  of 
iron  in  the  armature,  and  several  machines  of  this  kind,  with 
moving  or  stationary  conductors  were  constructed,  but  the  re- 
sults obtained  were  not  quite  satisfactory,  on  account  of  the 
great  mechanical  and  other  difficulties  encountered. 

The  study  of  the  properties  of  the  high  frequency  currents 
obtained  from  these  machines  is  very  interesting,  as  nearly  every 
experiment  discloses  something  new.  Two  coils  traversed  by 
such  a  current  attract  or  repel  each  other  with  a  force  which, 
owing  to  the  imperfection  of  our  sense  of  touch,  seems  contin- 
uous. An  interesting  observation,  already  noted  under  another 
form,  is  that  a  piece  of  iron,  surrounded  by  a  coil  through  which 
the  current  is  passing  appears  to  be  continuously  magnetized. 
This  apparent  continuity  might  be  ascribed  to  the  deficiency  of 
the  sense  of  touch,  but  there  is  evidence  that  in  currents  of  such 
high  frequencies  one  of  the  impulses  preponderates  over  the 
other. 

As  might  be  expected,  conductors  traversed  by  such  currents 
are  rapidly  heated,  owing  to  the  increase  of  the  resistance,  and 
the  heating  effects  are  relatively  much  greater  in  the  iron. 
The  hysteresis  losses  in  iron  are  so  great  that  an  iron  core, 
even  if  finely  subdivided,  is  heated  in  an  incredibly  short  time. 
To  give  an  idea  of  this,  an  ordinary  iron  wire  -^g-  inch  in 
diameter  inserted  within  a  coil  having  250  turns,  with  a  current 
estimated  to  be  five  amperes  passing  through  the  coil,  becomes 
within  two  seconds'  time  so  hot  as  to  scorch  wood.  Beyond  a 
certain  frequency,  an  iron  core,  no  matter  how  finely  subdivided, 
exercises  a  dampening  effect,  and  it  was  easy  to  find  a  point  at 


HIGH  FRJSQVKNCy  AND  HIGH  POTENTIAL  CURRENTS.      125 

whicli  tlie  impedance  <>f  a  coil  was  not  affected  by  the  presence 
of  a  core  consisting  of  a  bundle  of  very  thin  well  annealed  and 
varnished  iron  wires. 

Experiments  with  a  telephone,  a  conductor  in  a  strong  mag- 
netic field,  or  with  a  condenser  or  arc,  seem  to  afford  certain 
proof  that  sounds  far  above  the  usually  accepted  limit  of  hearing 
would  be  perceived  if  produced  with  sufficient  power.  The  arc 
produced  by  these  currents  possesses  several  interesting  features. 
Usually  it  emits  a  note  the  pitch  of  which  corresponds  to  twice 
the  frequency  of  the  current,  but  if  the  frequency  be  sufficiently 
high  it  becomes  noiseless,  the  limit  of  audition  being  determined 
principally  by  the  linear  dimensions  of  the  arc.  A  curious  fea- 
ture of  the  arc  is  its  persistency,  which  is  due  partly  to  the  in- 
ability of  the  gaseous  column  to  cool  and  increase  considerably 
in  resistance,  as  is  the  case  with  low  frequencies,  and  partly  to 
the  tendency  of  such  a  high  frequency  machine  to  maintain  a 
constant  current. 

In  connection  with  these  machines  the  condenser  affords  a  par- 
ticularly interesting  study.  Striking  effects  are  produced  by 
proper  adjustments  of  capacity  and  self-induction.  It  is  easy  to 
raise  the  electromotive  force  of  the  machine  to  many  times  the 
original  value  by  simply  adjusting  the  capacity  of  a  condenser 
connected  in  the  induced  circuit.  If  the  condenser  be  at  some 
distance  from  the  machine,  the  difference  of  potential  on  the 
terminals  of  the  latter  may  be  only  a  small  fraction  of  that  on 
the  condenser. 

But  the  most  interesting  experiences  are  gained  when  the  ten- 
sion of  the  currents  from  the  machine  is  raised  by  means  of  an 
induction  coil.  In  consequence  of  the  enormous  rate  of  change 
obtainable  in  the  primary  current,  much  higher  potential  differ- 
ences are  obtained  than  with  coils  operated  in  the  usual  ways, 
and,  owing  to  the  high  frequency,  the  secondary  discharge  pos- 
sesses many  striking  peculiarities.  Both  the  electrodes  behave 
generally  alike,  though  it  appears  from  some  observations  that 
one  current  impulse  preponderates  over  the  other,  as  before 
mentioned. 

The  physiological  effects  of  the  high  tension  discharge  are 
found  to  be  so  small  that  the  shock  of  the  coil  can  be  supported 
without  any  inconvenience,  except  perhaps  a  small  burn  produced 
by  the  discharge  upon  approaching  the  hand  to  one  of  the  ter- 
minals. The  decidedly  smaller  physiological  effects  of  these  cur- 


126  INVENTIONS  OF  NIKOLA  TESLA. 

rents  are  thought  to  be  due  either  to  a  different  distribution 
through  the  body  or  to  the  tissues  acting  as  condensers.  But  in 
the  case  of  an  induction  coil  with  a  great  many  turns  the  harmless- 
ness  is  principally  due  to  the  fact  that  but  little  energy  is  avail- 
able in  the  external  circuit  when  the  same  is  closed  through  the 
experimenter's  body,  on  account  of  the  great  impedance  of  the 
coil. 

In  varying  the  frequency  and  strenth  of  the  currents  through 
the  primary  of  the  coil,  the  character  of  the  secondary  discharge 
is  greatly  varied,  and  no  less  than  five  distincts  forms  are  ob- 
served :— A  weak,  sensitive  thread  discharge,  a  powerful  naming 
discharge,  and  three  forms  of  brush  or  streaming  discharges. 
Each  of  these  possesses  certain  noteworthy  features,  but  the  most 
interesting  to  study  are  the  latter. 

Under  certain  conditions  the  streams,  which  are  presumably 
due  to  the  violent  agitation  of  the  air  molecules,  issue  freely 
from  all  points  of  the  coil,  even  through  a  thick  insulation.  If 
there  is  the  smallest  air  space  between  the  primary  and  secondary, 
they  will  form  there  and  surely  injure  the  coil  by  slowly  warm- 
ing the  insulation.  As  they  form  even  with  ordinary  frequencies 
when  the  potential  is  excessive,  the  air-space  must  be  most  care- 
fully avoided.  These  high  frequency  streamers  differ  in  aspect 
and  properties  from  those  produced  by  a  static  machine.  The 
wind  produced  by  them  is  small  and  should  altogether  cease  if 
still  considerably  higher  frequencies  could  be  obtained.  A  pe- 
culiarity is  that  they  issue  as  freely  from  surfaces  as  from  points. 
(  hving  to  this,  a  metallic  vane,  mounted  in  one  of  the  terminals  of 
the  coil  so  as  to  rotate  freely,  and  having  one  of  its  sides  covered 
with  insulation,  is  spun  rapidly  around.  Such  a  vane  would  not 
rotate  with  a  steady  potential,  but  with  a  high  frequency  coil  it 
will  spin,  even  if  it  be  entirely  covered  with  insulation,  provided 
the  insulation  on  one  side  be  either  thicker  or  of  a  higher  specific 
inductive  capacity.  A  Crookes  electric  radiometer  is  also  spun 
around  when  connected  to  one  of  the  terminals  of  the  coil,  but 
only  at  very  high  exhaustion  or  at  ordinary  pressures. 

There  is  still  another  and  more  striking  peculiarity  of  such  a 
high  frequency  streamer,  namely,  it  is  hot.  The  heat  is  easily 
perceptible  with  frequencies  of  about  10,000,  even  if  the  poten- 
tial is  not  excessively  high.  The  heating  effect  is,  of  course,  due 
to  the  molecular  impacts  and  collisions.  Could  the  frequency 
and  potential  be  pushed  far  enough,  then  a  brush  could  be  pr«»- 


HIGH  FREQUENCY  AND  HIGH  POTENTIAL  CURRENTS.      127 

duced  resembling  in  every  particular  a  flame  and  giving  light 
and  heat,  jet  without  a  chemical  process  taking  place. 

The  hot  brush,  when  properly  produced,  resembles  a  jet  of 
burning  gas  escaping  under  great  pressure,  and  it  emits  an  extra- 
ordinary strong  smell  of  ozone.  The  great  ozonizing  action  is 
ascribed  to  the  fact  that  the  agitation  of  the  molecules  of  the  air 
is  more  violent  in  such  a  brush  than  in  the  ordinary  streamer  of 
a  static  machine.  But  the  most  powerful  brush  discharges  were 
produced  by  employing  currents  of  much  higher  frequencies  than 
it  was  possible  to  obtain  by  means  of  the  alternators.  These 
currents  were  obtained  by  disruptively  discharging  a  condenser 
and  setting  up  oscillations.  In  this  manner  currents  of  a  fre- 
quency of  several  hundred  thousand  were  obtained. 

Currents  of  this  kind,  Mr.  Tesla  pointed  out,  produce  striking 
effects.  At  these  frequencies,  the  impedance  of  a  copper  bar  is 
so  great  that  a  potential  difference  of  several  hundred  volts  can 
be  maintained  between  two  points  of  a  short  and  thick  bar,  and 
it  is  possible  to  keep  an  ordinary  incandescent  lamp  burning  at 
full  candle  power  by  attaching  the  terminals  of  the  lamp  to  two 
points  of  the  bar  no  more  than  a  few  inches  apart,  When  the 
frequency  is  extremely  high,  nodes  are  found  to  exist  on  such  a 
bar,  and  it  is  easy  to  locate  them  by  means  of  a  lamp. 

By  converting  the  high  tension  discharges  of  a  low  frequency 
coil  in  this  manner,  it  was  found  practicable  to  keep  a  few  lamps 
burning  on  the  ordinary  circuit  in  the  laboratory,  and  by  bring- 
ing the  undulation  to  a  low  pitch,  it  was  possible  to  operate  small 
motors. 

This  plan  likewise  allows  of  converting  high  tension  discharges 
of  one  direction  into  low  tension  unidirectional  currents,  by  ad- 
justing the  circuit  so  that  there  are  no  oscillations.  In  passing 
the  oscillating  discharges  through  the  primary  of  a  specially 
constructed  coil,  it  is  easy  to  obtain  enormous  potential  differences 
with  only  few  turns  of  the  secondary. 

Great  difficulties  were  at  first  experienced  in  producing  a  suc- 
cessful coil  on  this  plan.  It  was  found  necessary  to  keep  all  air, 
or  gaseous  matter  in  general,  away  from  the  charged  surfaces, 
and  oil  immersion  was  resorted  to.  The  wires  used  were  heavily 
covered  with  gutta-percha  and  wound  in  oil,  or  the  air  was  pumped 
out  by  means  of  a  Sprengel  pump.  The  general  arrangement 
was  the  following: — An  ordinary  induction  coil,  operated  from 
a  low  frequency  alternator,  was  used  to  charge  Leyden  jars.  The 


128  INVENTIONS  OF  NIKOLA  TESLA. 

jars  were  made  to  discharge  over  a  single  or  multiple  gap  through 
the  primary  of  the  second  coil.  To  insure  the  action  of  the  gap, 
the  arc  was  blown  out  by  a  magnet  or  air  blast.  To  adjust  the 
potential  in  the  secondary  a  small  oil  condenser  was  used,  or 
polished  brass  spheres  of  different  sizes  were  screwed  on  the 
terminals  and  their  distance  adjusted. 

When  the  conditions  were  carefully  determined  to  suit  each 
experiment,  magnificent  effects  were  obtained.  Two  wires, 
stretched  through  the  room,  each  being  connected  to  one  of  the 
terminals  of  the  coil,  emitted  streams  so  powerful  that  the  light 
from  them  allowed  distinguishing  the  objects  in  the  room  ;  the 
wires  became  luminous  even  though  covered  with  thick  and 
most  excellent  insulation.  When  two  straight  wires,  or  two  con- 
centric circles  of  wire,  are  connected  to  the  terminals,  and  set  at 
the  proper  distance,  a  uniform  luminous  sheet  is  produced  be- 
tween them.  It  was  possible  in  this  way  to  cover  an  ana  of 
more  than  one  meter  square  completely  with  the  streams.  By 
attaching  to  one  terminal  a  large  circle  of  wire  and  to  the  other 
terminal  a  small  sphere,  the  streams  are  focused  upon  the  sphere, 
produce  a  strongly  lighted  spot  upon  the  same,  and  present  the 
appearance  of  a  luminous  cone.  A  very  thin  wire  glued  upon  a 
plate  of  hard  rubber  of  great  thickness,  on  the  opposite  side  of 
which  is  fastened  a  tinfoil  coating,  is  rendered  intensely  luminous 
when  the  coating  is  connected  to  the  other  terminal  of  the  coil. 
Such  an  experiment  can  be  performed  also  with  low  frequency 
currents,  but  much  less  satisfactorily. 

When  the  terminals  of  such  a  coil,  even  of  a  very  small  one, 
are  separated  by  a  rubber  or  glass  plate,  the  discharge  spreads 
over  the  plate  in  the  form  of  streams,  threads  or  brilliant  sparks, 
and  affords  a  magnificent  display,  which  cannot  be  equaled  by 
the  largest  coil  operated  in  the  usual  ways.  By  a  simple  adjust- 
ment it  is  possible  to  produce  with  the  coil  a  succession  of  bril- 
liant sparks,  exactly  as  with  a  Holtz  machine. 

Under  certain  conditions,  when  the  frequency  of  the  oscillation 
is  very  great,  white,  phantom-like  streams  are  seen  to  break  forth 
from  the  terminals  of  the  coil.  The  chief  interesting  feature 
about  them  is,  that  they  stream  freely  against  the  outstretched 
hand  or  other  conducting  object  without  producing  any  sensa- 
tion, and  the  hand  may  be  approached  very  near  to  the  terminal 
without  a  spark  being  induced  to  jump.  This  is  due  presumably 
to  the  fact  that  a  considerable  portion  of  the  energy  is  carried 


HIGH  FREQUENCY  AND  HIGH  POTENTIAL  CURRENTS.      129 

away  or  dissipated  in  the  streamers,  and  the  difference  of  poten- 
tial between  the  terminal  and  the  hand  is  diminished. 

It  is  found  in  such  experiments  that  the  frequency  of  the 
vibration  and  the  quickness  of  succession  of  the  sparks  between 
the  knobs  affect  to  a  marked  degree  the  appearance  of  the 
streams.  When  the  frequency  is  very  low,  the  air  gives  way  in 
more  or  less  the  same  manner  as  by  a  steady  difference  of  poten- 
tial, and  the  streams  consist  of  distinct  threads,  generally  mingled 
with  thin  sparks,  which  probably  correspond  to  the  successive 
discharges  occurring  between  the  knobs.  But  when  the  fre- 
quency is  very  high,  and  the  arc  of  the  discharge  produces  a 
sound  which  is  loud  and  smooth  (which  indicates  both  that  oscil- 
lation takes  place  and  that  the  sparks  succeed  each  other  with 
great  rapidity),  then  the  luminous  streams  formed  are  perfectly 
uniform.  They  are  generally  of  a  purplish  hue,  but  when  the 
molecular  vibration  is  increased  by  raising  the  potential,  they  as- 
sume a  white  color. 

The  luminous  intensity  of  the  streams  increases  rapidly  when 
the  potential  is  increased;  and  with  frequencies  of  only  a  few 
hundred  thousand,  could  the  coil  be  made  to  withstand  a  suffi- 
ciently high  potential  difference,  there  is  no  doubt  that  the 
space  around  a  wire  could  be  made  to  emit  a  strong  light, 
merely  by  the  agitation  of  the  molecules  of  the  air  at  ordinary 
pressure. 

Such  discharges  of  very  high  frequency  which  render  lumi- 
nous the  air  at  ordinary  pressure  we  have  very  likely  occasion  to 
witness  in  the  aurora  borealis.  From  many  of  these  experi- 
ments it  seems  reasonable  to  infer  that  sudden  cosmic  disturb- 
ances, such  as  eruptions  on  the  sun,  set  the  electrostatic  charge 
of  the  earth  in  an  extremely  rapid  vibration,  and  produce  the 
glow  by  the  violent  agitation  of  the  air  in  the  upper  and  even  in 
the  lower  strata.  It  is  thought  that  if  the  frequency  were  low? 
or  even  more  so  if  the  charge  were  not  at  all  vibrating,  the 
lower  dense  strata  would  break  down  as  in  a  lightning  discharge. 
Indications  of  such  breaking  down  have  been  repeatedly  ob- 
served, but  they  can  be  attributed  to  the  fundamental  disturb- 
ances, which  are  few  in  number,  for  the  superimposed  vibration 
would  be  so  rapid  as  not  to  allow  a  disruptive  break. 

The  study  of  these  discharge  phenomena  has  led  Mr.  Tesla  to 
the  recognition  of  some  important  facts.  It  was  found,  as  already 
stated,  that  uascous  matter  must  be  most  carefully  excluded  from 


130  INVENTIONS  OP'  NIKOLA  TESLA. 

any  dielectric  which  is  subjected  to  great,  rapidly  changing  elec- 
trostatic stresses.  Since  it  is  difficult  to  exclude  the  gas  perfectly 
when  solid  insulators  are  used,  it  is  necessary  to  resort  to  liquid 
dielectrics.  When  a  solid  dielectric  is  used,  it  matters  little  how 
thick  and  how  good  it  is;  if  air  be  present,  streamers  form, 
which  gradually  heat  the  dielectric  and  impair  its  insulating 
power,  and  the  discharge  finally  breaks  through.  Under  ordi- 
nary conditions  the  best  insulators  are  those  which  possess  the 
highest  specific  inductive  capacity,  but  such  insulators  are  not 
the  best  to  employ  when  working  with  these  high  frequency 
currents,  for  in  most  cases  the  higher  specific  inductive  capacity 
is  rather  a  disadvantage.  The  prime  quality  of  the  insulating 
medium  for  these  currents  is  continuity.  For  this  reason  prin- 
cipally it  is  necessary  to  employ  liquid  insulators,  such  as  oils. 
If  two  metal  plates,  connected  to  the  terminals  of  the  coil,  are 
immersed  in  oil  and  set  a  distance  apart,  the  coil  may  be  kept 
working  for  any  length  of  time  without  a  break  occurring,  or 
without  the  oil  being  warmed,  but  if  air  bubbles  are  introduced, 
they  become  luminous ;  the  air  molecules,  by  their  impact 
against  the  oil,  heat  it,  and  after  some  time  cause  the  insulation 
to  give  way.  If,  instead  of  the  oil,  a  solid  plate  of  the  best 
dielectric,  even  several  times  thicker  than  the  oil  intervening 
between  the  metal  plates,  is  inserted  between  the  latter,  the  air 
having  free  access  to  the  charged  surfaces,  the  dielectric  i vari- 
ably is  warmed  and  breaks  down. 

The  employment  of  oil  is  advisable  or  necessary  even  with  low 
frequencies,  if  the  potentials  are  such  that  streamers  form,  but 
only  in  such  cases,  as  is  evident  from  the  theory  of  the  action. 
If  the  potentials  are  so  low  that  streamers  do  not  form,  then  it 
is  even  disadvantageous  to  employ  oil,  for  it  may,  principally  by 
confining  the  heat,  be  the  cause  of  the  breaking  down  of  the  in- 
sulation. 

The  exclusion  of  gaseous  matter  is  not  only  desirable  on  ac- 
count of  the  safety  of  the  apparatus,  but  also  on  account  of 
economy,  especially  in  a  condenser,  in  which  considerable  waste 
of  power  may  occur  merely  owing  to  the  presence  of  air,  if  the 
electric  density  on  the  charged  surfaces  is  great. 

In  the  course  of  these  investigations  a  phenomenon  of  special 
scientific  interest  was  observed.  It  may  be  ranked  among  the 
brush  phenomena,  in  fact  it  is  a  kind  of  brush  which  forms  at,  or 
near,  a  single  terminal  in  high  vacuum.  In  a  bulb  with  a  con- 


HIGH  FREQUENCY  AND  HIGH  POTENTIAL  CURRENTS.      131 

ducting  electrode,  even  if  the  latter  be  of  aluminum,  the  brush 
has  only  a  very  short  existence,  but  it  can  be  preserved  for  a  con- 
siderable length  of  time  in  a  bulb  devoid  of  any  conducting  elec- 
trode. To  observe  the  phenomenon  it  is  found  best  to  employ  a 
large  spherical  bulb  having  in  its  centre  a  small  bulb  supported 
on  a  tube  sealed  to  the  neck  of  the  former.  The  large  bulb  be- 
ing exhausted  to  a  high  degree,  and  the  inside  of  the  small  bulb 
being  connected  to  one  of  the  terminals  of  the  coil,  under  certain 
conditions  there  appears  a  misty  haze  around  the  small  bulb, 
which,  after  passing  through  some  stages,  assumes  the  form  of  a 
brush,  generally  at  right  angles  to  the  tube  supporting  the  small 
bulb.  When  the  brush  assumes  this  form  it  may  be  brought  to 
a  state  of  extreme  sensitiveness  to  electrostatic  and  magnetic  in- 
fluence. The  bulb  hanging  straight  down,  and  all  objects  being 
remote  from  it,  the  approach  of  the  observer  within  a  few  paces 
will  cause  the  brush  to  fly  to  the  opposite  side,  and  if  he  walks 
around  the  bulb  it  will  always  keep  on  the  opposite  side.  It  may 
begin  to  spin  around  the  terminal  long  before  it  reaches  that  sen- 
sitive stage.  When  it  begins  to  turn  around,  principally,  but 
also  before,  it  is  affected  by  a  magnet,  and  at  a  certain  stage  it  is 
susceptible  to  magnetic  influence  to  an  astonishing  degree.  A 
small  permanent  magnet,  with  its  poles  at  a  distance  of  no  more 
than  two  centimetres  will  affect  it  visibly  at  a  distance  of  two  me- 
tres, slowing  down  or  accelerating  the  rotation  according  to  how 
it  is  held  relatively  to  the  brush. 

When  the  bulb  hangs  with  the  globe  down,  the  rotation  is  al- 
ways clockwise.  In  the  southern  hemisphere  it  would  occur  in 
the  opposite  direction,  and  on  the  (magnetic)  equator  the  brush 
should  not  turn  at  all.  The  rotation  may  be  reversed  by  a  mag- 
net kept  at  some  distance.  The  brush  rotates  best,  seemingly, 
when  it  is  at  right  angles  to  the  lines  of  force  of  the  earth.  It, 
very  likely  rotates,  when  at  its  maximum  speed,  in  synchronism 
with  the  alternations,  say,  10,000  times  a  second.  The  rotation 
can  be  slowed  down  or  accelerated  by  the  approach  or  recession 
of  the  observer,  or  any  conducting  body,  but  it  cannot  be  re- 
versed by  putting  the  bulb  in  any  position.  Very  curious  experi- 
ments may  be  performed  with  the  brush  when  in  its  most  sensi- 
tive state.  For  instance,  the  brush  resting  in  one  position,  the 
experimenter  may,  by  selecting  a  proper  position,  approach  the 
hand  at  a  certain  considerable  distance  to  the  bulb,  and  he  may 
cjuisi'  the  brush  to  pass  oft  bv  merely  stiffening  the  muscles  of 


132  INVENTIONS  OF  NIKOLA  TESLA. 

the  arm,  the  mere  change  of  configuration  of  the  arm  and  the 
consequent  imperceptible  displacement  being  sufficient  to  disturb 
the  delicate  balance.  When  it  begins  to  rotate  slowly,  and  tin- 
hands  are  held  at  a  proper  distance,  it  is  impossible  to  make  even 
the  slightest  motion  without  producing  a  visible  effect  upon  the 
brush.  A  metal  plate  connected  to  the  other  terminal  of  the  coil 
affects  it  at  a  great  distance,  slowing  down  the  rotation  often  to 
one  turn  a  second. 

Mr.  Tesla  hopes  that  this  phenomenon  will  prove  a  valuable 
aid  in  the  investigation  of  the  nature  of  the  forces  acting  in  an 
electrostatic  or  magnetic  field.  If  there  is  any  motion  which  is 
measurable  going  on  in  the  space,  such  a  brush  would  be  apt  to 
reveal  it.  It  is,  so  to  speak,  a  beam  of  light,  frictionless,  devoid 
of  inertia.  On  account  of  its  marvellous  sensitiveness  to  electro- 
static or  magnetic  disturbances  it  may  be  the  means  of  sending 
signals  through  submarine  cables  with  any  speed,  and  even  of 
transmitting  intelligence  to  a  .distance  without  wires. 

In  operating  an  induction  coil  with  these  rapidly  alternating 
currents,  it  is  astonishing  to  note,  for  the  first  time,  the  great 
importance  of  the  relation  of  capacity,  self-induction,  and  fre- 
quency as  bearing  upon  the  general  result.  The  combined  effect 
of  these  elements  produces  many  curious  effects.  For  instance. 
two  metal  plates  are  connected  to  the  terminals  and  set  at  a  small 
distance,  so  that  an  arc  is  formed  between  them.  This  arc  />/v- 
vents  a  strong  current  from  flowing  through  the  coil.  If  the  art- 
be  interrupted  by  the  interposition  of  a  glass  plate,  the  capacity 
of  the  condenser  obtained  counteracts  the  self-induction,  and  a 
stronger  current  is  made  to  pass.  The  effects  of  capacity  are  the 
most  striking,  for  in  these  experiments,  since  the  self-induction 
and  frequency  both  are  high,  the  critical  capacity  is  very  small, 
and  need  be  but  slightly  varied  to  produce  a  very  considerable 
change.  The  experimenter  brings  his  body  in  contact  with  the 
terminals  of  the  secondary  of  the  coil,  or  attaches  to  one  or  both 
terminals  insulated  bodies  of  very  small  bulk,  such  as  exhausted 
bulbs,  and  he  produces  a  considerable  rise  or  fall  of  potential  on 
the  secondary,  and  greatly  affects  the  flow  of  the  current  through 
the  primary  coil. 

In  many  of  the  phenomena  observed,  the  presence  of  the  air, 
or,  generally  speaking,  of  a  medium  of  a  gaseous  nature  (using 
this  term  not  to  imply  specific  properties,  but  in  contradistinction 
to  homogeneity  or  perfect  continuity)  plays  an  important  part. 


HIGH  FREQUENCY  AND  HIGH  POTENTIAL  CURRENTS.      133 

as  it  allows  energy  to  be  dissipated  by  molecular  impact  or  bom- 
bardment. The  action  is  thus  explained:— When  an  insulated 
body  connected  to  a  terminal  of  the  coil  is  suddenly  charged  to 
high  potential,  it  acts  inductively  upon  the  surrounding  air,  or 
whatever  gaseous  medium  there  might  be.  The  molecules  or 
atoms  which  are  near  it  are,  of  course,  more  attracted,  and  move 
through  a  greater  distance  than  the  further  ones.  When  the 
nearest  molecules  strike  the  body  they  are  repelled,  and  collisions 
occur  at  all  distances  within  the  inductive  distance.  It  is  now 
clear  that,  if  the  potential  be  steady,  bat  little  loss  of  energy  can 
be  caused  in  this  way,  for  the  molecules  which  are  nearest  to 
the  body  having  had  an  additional  charge  imparted  to  them  by 
contact,  are  not  attracted  until  they  have  parted,  if  not  with  all, 
at  least  with  most  of  the  additional  charge,  which  can  be  accom- 
plished only  after  a  great  many  collisions.  This  is  inferred  from 
the  fact  that  with  a  steady  potential  there  is  but  little  loss  in  dry 
air.  When  the  potential,  instead  of  being  steady,  is  alternating, 
the  conditions  are  entirely  different.  In  this  case  a  rhythmical 
bombardment  occurs,  no  matter  whether  the  molecules  after 
coming  in  contact  with  the  body  lose  the  imparted  charge  or 
not,  and,  what  is  more,  if  the  charge  is  not  lost,  the  impacts  are 
all  the  more  violent.  Still,  if  the  frequency  of  the  impulses 
be  very  small,  the  loss  caused  by  the  impacts  and  collisions  would 
not  be  serious  unless  the  potential  was  excessive.  But  when 
extremely  high  frequencies  and  more  or  less  high  potentials  are 
used,  the  loss  may  be  very  great,  The  total  energy  lost  per  unit 
of  time  is  proportionate  to  the  product  of  the  number  of  impacts 
per  second,  or  the  frequency  and  the  energy  lost  in  each  impact. 
But  the  energy  of  an  impact  must  be  proportionate  to  the  square 
of  the  electric  density  of  the  body,  on  the  assumption  that  the 
charge  imparted  to  the  molecule  is  proportionate  to  that  density. 
It  is  concluded  from  this  that  the  total  energy  lost  must  be  pro- 
portionate to  the  product  of  the  frequency  and  the  square  of  the 
electric  density;  but  this  law  needs  experimental  confirmation. 
Assuming  the  preceding  considerations  to  be  true,  then,  by  ra- 
pidly alternating  the  potential  of  a  body  immersed  in  an  insulat- 
ing gaseous  medium,  any  amount  of  energy  may  be  dissipated 
into  space.  Most  of  that  energy,  then,  is  not  dissipated  in  the 
form  of  long  ether  waves,  propagated  to  considerable  distance, 
as  is  thought  most  generally,  but  is  consumed  in  impact  and 
collisional  losses — that  is,  heat  vibrations — on  the  surface  and  in 


134  INVENTIONS  OF  NIKOLA  TESLA. 

the  vicinity  of  the  body.  To  reduce  the  dissipation  it  is  neces- 
sary to  work  with  a  small  electric  density — the  smaller,  the 
higher  the  frequency. 

The  behavior  of  a  gaseous  medium  to  such  rapid  alternations 
of  potential  makes  it  appear  plausible  that  electrostatic  dis- 
turbances of  the  earth,  produced  by  cosmic  events,  may  have 
great  influence  upon  the  meteorological  condition^.  When  such 
disturbances  occur  both  the  frequency  of  the  vibrations  of  the 
charge  and  the  potential  are  in  all  probability  excessive,  and  the 
energy  converted  into  heat  may  be  considerable.  Since  the 
density  must  be  unevenly  distributed,  either  in  consequence  of 
the  irregularity  of  the  earth's  surface,  or  on  account  of  the 
condition  of  the  atmosphere  in  various  places,  the  effect  pro- 
duced would  accordingly  vary  from  place  to  place.  Considerable 
variations  in  the  temperature  and  pressure  of  the  atmosphere 
may  in  this  manner  be  caused  at  any  point  of  the  surface  of  the 
earth.  The  variations  may  be  gradual  or  very  sudden,  according 
to  the  nature  of  the  original  disturbance,  and  may  produce  rain 
and  storms,  or  locally  modify  the  weather  in  any  way. 

From  many  experiences  gathered  in  the  course  of  these  inves- 
tigations it  appears  certain  that  in  lightning  discharges  the  air  is 
an  element  of  importance.  For  instance,  during  a  storm  a 
stream  may  form  on  a  nail  or  pointed  projection  of  a  building. 
If  lightning  strikes  somewhere  in  the  neighborhood* the  harm- 
less static  discharge  may,  in  consequence  of  the  oscillations  set 
up,  assume  the  character  of  a  high-frequency  streamer,  and  the 
nail  or  projection  may  be  brought  to  a  high  temperature  by  the 
violent  impact  of  the  air  molecules.  Thus,  it  is  thought,  a 
building  may  be  set  on  fire  without  the  lightning  striking  it.  In 
like  manner  small  metallic  objects  may  be  fused  and  volatilized 
— as  frequently  occurs  in  lightning  discharges — merely  because 
they  are  surrounded  by  air.  Were  they  immersed  in  a  practi- 
cally continuous  medium,  such  as  oil,  they  would  probably  be 
safe,  as  the  energy  would  have  to  spend  itself  elsewhere. 

An  instructive  experience  having  a  bearing  on  this  subject  is 
the  following: — A  glass  tube  of  an  inch  or  so  in  diameter  and 
several  inches  long  is  taken,  and  a  platnium  wire  sealed  into  it, 
the  wire  running  through  the  center  of  the  tube  from  end  to 
end.  The  tube  is  exhausted  to  a  moderate  degree.  If  a  steady 
current  is  passed  through  the  wire  it  is  heated  uniformly  in  all 
parts  and  the  gas  in  the  tube  is  of  no  consequence.  But  if  high 


HIGH  FREQUENCY  AND  HIGH  POTENTIAL  CURRENT*.      135 

frequency  discharges  are  directed  through  the  wire,  it  is  heated 
more  on  the  ends  than  in  the  middle  portion,  and  if  the  fre- 
quency, or  rate  of  charge,  is  high  enough,  the  wire  might  as 
well  be  cut  in  the  middle  as  not,  for  most  of  the  heating  on  the 
ends  is  due  to  the  rarefied  gas.  Here  the  gas  might  only  act  as 
a  conductor  of  no  impedance,  diverting  the  current  from  the 
wire  as  the  impedance  of  the  latter  is  enormously  increased,  and 
merely  heating  the  ends  of  the  wire  by  reason  of  their  resistance 
to  the  passage  of  the  discharge.  But  it  is  not  at  all  necessary  that 
the  gas  in  the  tube  should  he  conducting ;  it  might  be  at  an  ex- 
tremely low  pressure,  still  the  ends  of  the  wire  would  be  heated  ; 
however,  as  is  ascertained  by  experience,  only  the  two  ends 
would  in  such  case  not  be  electrically  connected  through  the 
gaseous  medium.  Now,  what  with  these  frequencies  and  poten- 
tials occurs  in  an  exhausted  tube,  occurs  in  the  lightning  discharge 
at  ordinary  pressure. 

From  the  facility  with  which  any  amount  of  energy  may  be 
carried  off  through  a  gas,  Mr.  Tesla  infers  that  the  best  wTay  to 
render  harmless  a  lightning  discharge  is  to  afford  it  in  some  way 
a  passage  through  a  volume  of  gas. 

The  recognition  of  some  of  the  above  facts  has  a  bearing  upon 
far-reaching  scientific  investigations  in  which  extremely  high 
frequencies  and  potentials  are  used.  In  such  cases  the  air  is  an 
important  factor  to  be  considered.  So,  for  instance,  if  two  wires 
are  attached  to  the  terminals  of  the  coil,  and  the  streamers  issue 
from'  them,  there  is  dissipation  of  energy  in  the  form  of  heat 
and  light,  and  the  wires  behave  like  a  condenser  of  larger  capac- 
ity. If  the  wires  be  immersed  in  oil,  the  dissipation  of  energy 
is  prevented,  or  at  least  reduced,  and  the  apparent  capacity  is 
diminished.  The  action  of  the  air  would  seem  to  make  it  very 
difficult  to  tell,  from  the  measured  or  computed  capacity  of  a 
condenser  in  which  the  air  is  acted  upon,  its  actual  capacity  or 
vibration  period,  especially  if  the  condenser  is  of  very  small  sur- 
face and  is  charged  to  a  very  high  potential.  As  many  import- 
ant results  are  dependant  upon  the  correctness  of  the  estimation 
of  the  vibration  period,  this  subject  demands  the  most  careful 
scrutiny  of  investigators. 

In  Leyden  jars  the  loss  due  to  the  presence  of  air  is  compara- 
tively small,  principally  on  account  of  the  great  surface  of  the 
coatings  and  the  small  external  action,  but  if  there  are  streamers 
on  the  top,  the  loss  may  be  considerable,  and  the  period  of  vibra- 


136  INVENTIONS  OF  NIKOLA  TESLA. 

tion  is  affected.  In  a  resonator,  the  density  is  small,  but  the 
frequency  is  extreme,  and  may  introduce  a  considerable  error. 
It  appears  certain,  at  any  rate,  that  the  periods  of  vibration  of  a 
charged  body  in  a  gaseous  and  in  a  continuous  medium,  such 
as  oil,  are  different,  on  account  of  the  action  of  the  former,  as 
explained. 

Another  fact  recognized,  which  is  of  some  consequence,  is, 
that  in  similar  investigations  the  general  considerations  of  static 
screening  are  not  applicable  when  a  gaseous  medium  is  present. 
This  is  evident  from  the  following  experiment : — A  short  and 
wide  glass  tube  is  taken  and  covered  with  a  substantial  coating  of 
bronze  powder,  barely  allowing  the  light  to  shine  a  little  through. 
The  tube  is  highly  exhausted  and  suspended  on  a  metallic  clasp 
from  the  end  of  a  wire.  When  the  wire  is  connected  with  one 
of  the  terminals  of  the  coil,  the  gas  inside  of  the  tube  is  lighted 
in  spite  of  the  metal  coating.  Here  the  metal  evidently  does 
not  screen  the  gas  inside  as  it  ought  to,  even  if  it  be  very  thin 
and  poorly  conducting.  Yet,  in  a  condition  of  rest  the  metal 
coating,  however  thin,  screens  the  inside  perfectly. 

One  of  the  most  interesting  results  arrived  at  in  pursuing  these 
experiments,  is  the  demonstration  of  the  fact  that  a  gaseous  me- 
dium, upon  which  vibration  is  impressed  by  rapid  changes  of 
electrostatic  potential,  is  rigid.  In  illustration  of  this  result  an 
experiment  made  by  Mr.  Tesla  may  by  cited  : — A  glass  tube  about 
one  inch  in  diameter  and  three  feet  long,  with  outside  condenser 
coatings  on  the  ends,  was  exhausted  to  a  certain  point,  when,  the 
tube  being  suspended  freely  from  a  wire  connecting  the  upper  coat- 
ing to  one  of  the  terminals  of  the  coil,  the  discharge  appeared  in 
the  form  of  a  luminous  thread  passing  through  the  axis  of  the  tube. 
Usually  the  thread  was  sharply  defined  in  the  upper  part  of  the 
tube  and  lost  itself  in  the  lower  part.  When  a  magnet  or  the 
finger  was  quickly  passed  near  the  upper  part  of  the  luminous 
thread,  it  was  brought  out  of  position  by  magnetic  or  electro- 
static influence,  and  a  transversal  vibration  like  that  of  a  sus- 
pended cord,  with  one  or  more  distinct  nodes,  was  set  up,  which 
lasted  for  a  few  minutes  and  gradually  died  out.  By  suspending 
from  the  lower  condenser  coating  metal  plates  of  different  sizes, 
the  speed  of  the  vibration  was  varied.  This  vibration  would 
seem  to  show  beyond  doubt  that  the  thread  possessed  rigidity, 
at  least  to  transversal  displacements. 

Many  experiments  were  tried  to  demonstrate  this  property  in 


HIGH  FREQUENCY  AND  HIGH  POTENTIAL  CURRENTS.      137 

air  at  ordinary  pressure.  Though  no  positive  evidence  has  been 
obtained,  it  is  thought,  nevertheless,  that  a  high  frequency  brush 
or  streamer,  if  the  frequency  could  be  pushed  far  enough,  would 
be  decidedly  rigid.  A  small  sphere  might  then  be  moved  within 
it  quite  freely,  but  if  tin-own  against  it  the  sphere  would  rebound. 
An  ordinary  flame  cannot  possess  rigidity  to  a  marked  degree 
because  the  vibration  is  directionless ;  but  an  electric  arc,  it  is 
believed,  must  possess  that  property  more  or  less.  A  luminous 
band  excited  in  a  bulb  by  repeated  discharges  of  a  Leyden  jar 
must  also  possess  rigidity,  and  if  deformed  and  suddenly  released 
should  vibrate. 

From  like  considerations  other  conclusions  of  interest  are 
readied.  The  most  probable  medium  filling  the  space  is  one 
consisting  of  independent  carriers  immersed  in  an  insulating 
fluid.  If  through'  this  medium  enormous  electrostatic  stresses 
are  assumed  to  act,  which  vary  rapidly  in  intensity,  it  would 
allow  the  motion  of  a  body  through  it,  yet  it  would  be  rigid  and 
elastic,  although  the  fluid  itself  might  be  devoid  of  these  pro- 
perties. Furthermore,  on  the  assumption  that  the  independent 
carriers  are  of  any  configuration  such  that  the  fluid  resistance  to 
motion  in  one  direction  is  greater  than  in  another,  a  stress  of 
that  nature  would  cause  the  carriers  to  arrange  themselves  in 
groups,  since  they  would  turn  to  each  other  their  sides  of  the 
greatest  electric  density,  in  which  position  the  fluid  resistance  to 
approach  would  be  smaller  than  to  receding.  If  in  a  medium  of 
the  above  characteristics  a  brush  would  be  formed  by  a  steady 
potential,  an  exchange  of  the  carriers  would  go  on  continually, 
and  there  would  be  less  carriers  per  unit  of  volume  in  the  brush 
than  in  the  space  at  some  distance  from  the  electrode,  this  cor- 
responding to  rarefaction.  If  the  potential  were  rapidly  chang- 
ing, the  result  would  be  very  different ;  the  higher  the  freqency 
of  the  pulses,  the  slower  would  be  the  exchange  of  the  carriers ; 
finally,  the  motion  of  translation  through  measurable  space  would 
cease,  and,  with  a  sufficiently  high  frequency  and  intensity  of  the 
stress,  the  carriers  would  be  drawn  towards  the  electrode,  and 
compression  would  result. 

An  interesting  feature  of  these  high  frequency  currents  is  that 
they  allow  of  operating  all  kinds  of  devices  by  connecting  the  de- 
vice with  only  one  leading  wire  to  the  electric  source.  In  fact, 
under  certain  conditions  it  may  be  more  economical  to  supply  the 
electrical  energy  witli  one  lead  than  with  two. 


138  INVENTIONS  OF  NIKOLA  TESLA. 

An  experiment  of  special  interest  shown  by  Mr.  Tesla,  is  the 
running,  by  the  use  of  only  one  insulated  line,  of  a  motor  oper- 
ating on  the  principle  of  the  rotating  magnetic  field  enunciated 
by  Mr.  Tesla.  A  simple  form  of  such  a  motor  is  obtained  by 
winding  upon  a  laminated  iron  core  a  primary  and  close  to  it  a 
secondary  coil,  closing  the  ends  of  the  latter  and  placing  a  freely 
movable  metal  disc  within  the  influence  of  the  moving  field. 
The  secondary  coil  may,  however,  be  omitted.  When  one  of  the 
ends  of  the  primary  coil  of  the  motor  is  connected  to  one  of  the 
terminals  of  the  high  frequency  coil  arid  the  other  end  to  an 
insulated  metal  plate,  which,  it  should  be  stated,  is  not  absolutely 
necessary  for  the  success  of  the  experiment,  the  disc  is  set  in 
rotation. 

Experiments  of  this  kind  seem  to  bring  it  within  possibility  to 
operate  a  motor  at  any  point  of  the  earth's  surface  from  a  cen- 
tral source,  without  any  connection  to  the  same  except  through 
the  earth.  If,  by  means  of  powerful  machinery,  rapid  variations 
of  the  earth's  potential  were  produced,  a  grounded  wire  reaching 
up  to  some  height  would  be  traversed  by  a  current  which  could 
be  increased  by  connecting  the  free  end  of  the  wire  to  a  body  of 
some  size.  The  current  might  be  converted  to  low  tension  and 
used  to  operate  a  motor  or  other  device.  The  experiment,  which 
would  be  one  of  great  scientific  interest,  would  probably  best 
succeed  on  a  ship  at  sea.  In  this  manner,  even  if  it  were  not 
possible  to  operate  machinery,  intelligence  might  be  transmitted 
quite  certainly. 

In  the  course  of  this  experimental  study  special  attention  was 
devoted  to  the  heating  effects  produced  by  these  currents,  which 
are  not  only  striking,  but  open  up  the  possibility  of  producing  a 
more  efficient  illumiuant.  It  is  sufficient  to  attach  to  the  coil 
terminal  a  thin  wire  or  filament,  to  have  the  temperature  of  the 
latter  perceptibly  raised.  If  the  wire  or  filament  be  enclosed  in 
a  bulb,  the  heating  effect  is  increased  by  preventing  the  circula- 
tion of  the  air.  If  the  air  in  the  bulb  be  strongly  compressed, 
the  displacements  are  smaller,  the  impacts  less  violent,  and  the 
heating  effect  is  diminished.  On  the  contrary,  if  the  air  in  the 
bulb  be  exhausted,  an  inclosed  lamp  filament  is  brought  to  in- 
candescence, and  any  amount  of  light  may  thus  be  produced. 

The  heating  of  the  inclosed  lamp  filament  depends  on  so 
many  things  of  a  different  nature,  that  it  is  difficult  to  give  a 
generally  applicable  rule  under  which  the  maximum  heating 


ITIGH  FREQUENCY  AND  HTGH  POTENTIAL  CURRENTS.      139 

occurs.  As  regards  the  size  of  the  bull),  it  is  ascertained  that  at 
ordinary  or  only  slightly  differing  atmospheric  pressures,  when 
air  is  a  good  insulator,  the  filament  is  heated  more  in  a  small 
bulb,  because  of  the  better  confinement  of  heat  in  this  case.  At 
lower  pressures,  when  air  becomes  conducting,  the  heating  ef- 
fect is  greater  in  a  large  bull),  but  at  excessively  high  degrees  of 
exhaustion  there  seems  to  be,  beyond  a  certain  and  rather  small 
size  of  the  vessel,  no  perceptible  difference  in  the  heating. 

The  shape  of  the  vessel  is  also  of  some  importance,  and  it  has 
been  found  of  advantage  for  reasons  of  economy  to  employ  a 
spherical  bulb  with  the  electrode  mounted  in  its  centre,  where 
the  rebounding  molecules  collide. 

It  is  desirable  on  account  of  economy  that  all  the  energy  sup- 
plied to  the  bulb  from  the  source  should  reach  without  loss  the 
body  to  be  heated.  The  loss  in  conveying  the  energy  from  the 
source  to  the  body  may  be  reduced  by  employing  thin  wires 
heavily  coated  with  insulation,  and  by  the  use  of  electrostatic 
screens.  It  is  to  be  remarked,  that  the  screen,  cannot  be  con- 
nected to  the  ground  as  under  ordinary  conditions. 

In  the  bulb  itself  a  large  portion  of  the  energy' supplied  may 
be  lost  by  molecular  bombardment  against  the  wire  connecting 
the  body  to  be  heated  with  the  source.  Considerable  improve- 
ment was  effected  by  covering  the  glass  stem  containing  the  wire 
with  a  closely  fitting  conducting  tube.  This  tube  is  made  to 
project  a  little  above  the  glass,  and  prevents  the  cracking  of  the 
latter  near  the  heated  body.  The  effectiveness  of  the  conducting 
tube  is  limited  to  very  high  degrees  of  exhaustion.  It  diminishes 
the  energy  lost  in  bombardment  for  two  reasons;  first,  the 
charge  given  up  by  the  atoms  spreads  over  a  greater  area,  and 
hence  the  electric  density  at  any  point  is  small,  and  the  atoms 
are  repelled  with  less  energy  than  if  they  would  strike  against  a 
good  insulator;  secondly,  as  the  tube  is  electrified  by  the  atoms 
which  first  come  in  contact  with  it,  the  progress  of  the  following- 
atoms  against  the  tube  is  more  or  less  checked  by  the  repulsion 
which  the  electrified  tube  must  exert  upon  the  similarly  electrified 
atoms.  This,  it  is  thought,  explains  why  the  discharge  through 
a  bulb  is  established  with  much  greater  facility  when  an  insulator, 
than  when  a  conductor,  is  present. 

During  the  investigations  a  great  many  bulbs  of  different  con- 
struction, with  electrodes  of  different  material,  were  experimented 
upon,  and  a  number  of  observations  of  interest  were  made.  Mr. 


140  INVENTIONS  OF  NIKOLA   TESLA. 

Tesla  has  found  tlmt  the  deterioration  of  the  electrode  is  the  less, 
the  higher  the  frequency.  This  was  to  be  expected,  as  then  the 
heating  is  effected  by  many  small  impacts,  instead  by  fewer  and 
more  violent  ones,  which  quickly  shatter  the  structure.  The  de- 
terioration is  also  smaller  when  the  vibration  is  harmonic.  Thus 
an  electrode,  maintained  at  a  certain  degree  of  heat,  lasts  much 
longer  with  currents  obtained  from  an  alternator,  than  with 
those  obtained  by  means  of  a  disruptive  discharge.  One  of  the 
most  durable  electrodes  was  obtained  from  strongly  compressed 
carborundum,  which  is  a  kind  of  carbon  recently  produced  by 
Mr.  E.  G.  Acheson,  of  Monongahela  City,  Pa.  From  experi- 
ence, it  is  inferred,  that  to  be  most  durable,  the  electrode  should 
be  in  the  form  of  a  sphere  with  a  highly  polished  surface. 

In  some  bulbs  refractory  bodies  were  mounted  in  a  carbon  cup 
and  put  under  the  molecular  impact.  It  was  observed  in 
such  experiments  that  the  carbon  cup  was  heated  at  first,  until  a 
higher  temperature  was  reached;  then  most  of  the  bombard- 
ment was  directed  against  the  refractory  body,  and  the  carbon 
was  relieved.  In  general,  when  different  bodies  were  mounted 
in  the  bulb,  the  hardest  fusible  would  be  relieved,  and  would 
remain  at  a  considerably  lower  temperature.  This  was  necessi- 
tated by  the  fact  that  most  of  the  energy  supplied  would  find 
its  way  through  the  body  \vhioh  was  more  easily  fused  or  "evap- 
orated." 

Curiously  enough  it  appeared  in  some  of  the  experiments 
made,  that  a  body  was  fused  in  a  bulb  under  the  molecular  im- 
pact by  evolution"  of  less  light  than  when  fused  by  the  applica- 
tion of  heat  in  ordinary  ways.  This  may  be  ascribed  to  a 
loosening  of  the  structure  of  the  body  under  the  violent  impacts 
and  changing  stresses. 

Some  experiments  seem  to  indicate  that  under  certain  condi- 
tions a  body,  conducting  or  nonconducting,  may,  when  bom- 
barded, emit  light,  which  to  all  appearances  is  due  to  phosphor- 
escence, but  may  in  reality  be  caused  by  the  incandescence  of  an 
infinitesimal  layer,  the  mean  temperature  of  the  body  being 
comparatively  small.  Such  might  be  the  case  if  each  single 
rhythmical  impact  were  capable  of  instantaneously  exciting  the 
retina,  and  the  rhythm  were  just  high  enough  to  cause  a  continuous 
impression  in  the  eye.  According  to  this  view,  a  coil  operated 
by  disruptive  discharge  would  be  eminently  adapted  to  produce 
such  a  result,  and  it  is  found  by  experience  that  its  power  of 


HIGH  FREQUENCY  AND  HIGH  POTENTIAL  CURRENTS.      141 

exciting  phosphorescence  is  extraordinarily  great.  It  is  capable 
of  exciting  phosphorescence  at  comparatively  low  degrees  of 
exhaustion,  and  also  projects  shadows  at  pressures  far  greater 
than  those  at  which  the  mean  free  path  is  comparable  to  the 
dimensions  of  the  vessel.  The  latter  observation  is  of  some  im- 
portance, inasmuch  as  it  may  modify  the  generally  accepted  views 
in  regard  to  the  "radiant  state"  phenomena. 

A  thought  which  early  and  naturally  suggested  itself  to  JVI  r. 
Tesla,  was  to  utilize  the  great  inductive  effects  of  high  frequency 
currents  to  produce  light  in  a  sealed  glass  vessel  without  the  use 
of  leading  in  wires.  Accordingly,  many  bulbs  were  constructed 
in  which  the  energy  necessary  to  maintain  a  button  or  filament 
at  high  incandescence,  was  supplied  through  the  glass  by  either 
electrostatic  or  electrodynamic  induction.  It  was  easy  to  regu- 
late the  intensity  of  the  light  emitted  by  means  of  an  externally 
applied  condenser  coating  connected  to  an  insulated  plate,  or 
simply  by  means  of  a  plate  attached  to  the  bulb  which  at  the 
same  time  performed  the  function  of  a  shade. 

A  subject  of  experiment,  which  has  been  exhaustively  treated 
in  England  by  Prof.  J.  J.  Thomson,  has  been  followed  up  inde- 
pendently by  Mr.  Tesla  from  the  beginning  of  this  study,  namely, 
to  excite  by  electrodynamic  induction  a  luminous  band  in  a  closed 
tube  or  bulb.  In  observing  the  behavior  of  gases,  and  the 
luminous  phenomena  obtained,  the  importance  of  the  electro- 
static effects  was  noted  and  it  appeared  desirable  to  produce 
enormous  potential  differences,  alternating  with  extreme  rapidity. 
Experiments  in  this  direction  led  to  some  of  the  most  interest- 
ing results  arrived  at  in  the  course  of  these  investigations.  It 
was  found  that  by  rapid  alternations  of  a  high  electrostatic  po- 
tential, exhausted  tubes  could  be  lighted  at  considerable  distances 
from  a  conductor  connected  to  a  properly  constructed  coil,  and 
that  it  was  practicable  to  establish  with  the  coil  an  alternating 
electrostatic  field,  acting  through  the  whole  room  and  lighting  a 
tube  wherever  it  was  placed  within  the  four  walls.  Phosphores- 
cent bulbs  may  be  excited  in  such  a  field,  and  it  is  easy  to  regu- 
late the  effect  by  connecting  to  the  bulb  a  small  insulated  metal 
plate.  It  was  likewise  possible  to  maintain  a  filament  or  button 
mounted  in  a  tube  at  bright  incandescence,  and,  in  one  experi- 
ment, a  mica  vane  was  spun  by  the  incandescence  of  a  platinum 
wire. 

Coming  now  to  the  lecture  delivered  in  Philadelphia  and   St. 


142  INVENTIONS  OF  NIKOLA  TESLA. 

Louis,  it  may  be  remarked  that  to  the  superficial  reader,  Mr. 
Tesla's  introduction,  dealing  with  the  importance  of  the  eye,  might 
appear  as  a  digression,  but  the  thoughtful  reader  will  find  therein 
much  food  for  meditation  and  speculation.  Throughout  his  dis- 
course one  can  trace  Mr.  Tesla's  effort  to  present  in  a  popular 
way  thoughts  and  views  on  the  electrical  phenomena  which  have 
in  recent  years  captivated  the  scientific  world,  but  of  which  the 
general  public  has  even  yet  merely  received  an  inkling.  Mr. 
Tesla  also  dwells  rather  extensively  on  his  well-known  method  of 
high-frequency  conversion ;  and  the  large  amount  of  detail  in- 
formation will  be  gratefully  received  by  students  and  experi- 
menters in  this  virgin  field.  The  employment  of  apt  analogies 
in  explaining  the  fundamental  principles  involved  makes  it  easy 
for  all  to  gain  a  clear  idea  of  their  nature.  Again,  the  ease  with 
which,  thanks  to  Mr.  Tesla's  efforts,  these  high-frequency  cur- 
rents may  now  be  obtained  from  circuits  carrying  almost  any 
kind  of  current,  cannot  fail  to  result  in  an  extensive  broadening 
of  this  field  of  research,  which  offers  so  many  possibilities.  M  r. 
Tesla,  true  philosopher  as  he  is,  does  not  hesitate  to  point  out 
defects  in  some  of  his  methods,  and  indicates  the  lines  which  t<> 
him  seem  the  most  promising.  Particular  stress  is  laid  by  him 
upon  the  employment  of  a  medium  in  which  the  discharge 
electrodes  should  be  immersed  in  order  that  this  method  of  con- 
version may  be  brought  to  the  highest  perfection.  He  has  evi- 
dently taken  pains  to  give  as  much  useful  information  as  possible 
to  those  who  wish  to  follow  in  his  path,  as  he  shows  in  detail  the 
circuit  arrangements  to  be  adopted  in  all  ordinary  cases  met  with 
in  practice,  and  although  some  of  these  methods  were  described 
by  him  two  years  before,  the  additional  information  is  still  timely 
and  welcome. 

In  his  experiments  he  dwells  first  on  some  phenomena  pro- 
duced by  electrostatic  force,  which  he  considers  in  the  light  of 
modern  theories  to  be  the  most  important  force  in  nature  for  us 
to  investigate.  At  the  very  outset  he  shows  a  strikingly  novel 
experiment  illustrating  the  effect  of  a  rapidly  varying  electrosta- 
tic force  in  a  gaseous  medium,  by  touching  with  one  hand  one  of 
the  terminals  of  a  200,000  volt  transformer  and  bringing  tin- 
other  hand  to  the  opposite  terminal.  The  powerful  streamers 
which  issued  from  his  hand  and  astonished  his  audiences  formed 
a  capital  illustration  of  some  of  the  views  advanced,  and  afforded 
Mr.  Tesla  an  opportunity  of  pointing  out  the  true  reasons  why. 


HIGH  FREQUENCY  AND  HIGH  POTENTIAL  CURRENTS.      143 

with  these  currents,  such  an  amount  of  energy  can  be  passed 
through  the  body  with  impunity.  He  then  showed  by  experi- 
ment the  difference  between  a  steady  and  a  rapidly  varying  force 
upon  the  dielectric.  This  difference  is  most  strikingly  illustrated 
in  the  experiment  in  which  a  bulb  attached  to  the  end  of  a  wire 
in  connection  with  one  of  the  terminals  of  the  transformer  is 
ruptured,  although  all  extraneous  bodies  are  remote  from  the 
bulb.  He  next  illustrates  how  mechanical  motions  are  produced 
by  a  varying  electrostatic  force  acting  through  a  gaseous  medium. 
The  importance  of  the  action  of  the  air  is  particularly  illustrated 
by  an  interesting  experiment. 

Taking  up  another  class  of  phenomena,  namely,  those  of  dyna- 
mic electricity,  Mr.  Tesla  produced  in  a  number  of  experiments 
a  variety  of  effects  by  the  employment  of  only  a  single  wire 
with  the  evident  intent  of  impressing  upon  his  audience  the  idea 
that  electric  vibration  or  current  can  be  transmitted  witli  ease, 
without  any  return  circuit ;  also  how  currents  so  transmitted  can 
be  converted  and  used  for  many  practical  purposes.  A  number 
of  experiments  are  then  shown,  illustrating  the  effects  of  fre- 
quency, self-induction  and  capacity;  then  a  number  of  ways  of 
operating  motive  and  other  devices  by  the  use  of  a  single  lead. 
A  number  of  novel  impedance  phenomena  are  also  shown  which 
cannot  fail  to  arouse  interest. 

Mr.  Tesla  next  dwelt  upon  a  subject  which  he  thinks  of  great 
importance,  that  is,  electrical  resonance,  which  he  explained  in  a 
popular  way.  He  expressed  his  firm  conviction  that  by  observ- 
ing proper  conditions,  intelligence,  and  possibly  even  power,  can 
be  transmitted  through  the  medium  or  through  the  earth;  and 
he  considers  this  problem  worthy  of  serious  and  immediate  con- 
sideration. 

Coming  now  to  the  light  phenomena  in  particular,  lie  illustrated 
the  four  distinct  kinds  of  these  phenomena  in  an  original  way, 
which  to  many  must  have  been  a  revelation.  Mr.  Tesla  attributes 
these  light  effects  to  molecular  or  atomic  impacts  produced  by  a 
varying  electrostatic  stress  in  a  gaseous  medium.  Fie  illustrated 
in  a  series  of  novel  experiments  the  effect  of  the  gas  surround- 
ing the  conductor  and  shows  beyond  a  doubt  that  with  high  fre- 
quency and  high  potential  currents,  the  surrounding  gas  is  of 
paramount  importance  in  the  heating  of  the  conductor.  He 
attributes  the  heating  partially  to  a  conduction  current  and  par- 
tially to  bombardment,  and  demonstrates  that  in  manv  cases  the 


144  INVENTIONS  OF  NIKOLA  TESLA. 

heating  may  be  practically  due  to  the  bombardment  alone.  He 
pointed  out  also  that  the  skin  effect  is  largely  modi  lied  by  the 
presence  of  the  gas  or  of  an  atomic  medium  in  general.  He 
showed  also  some  interesting  experiments  in  which  the  effect  of 
convection  is  illustrated.  Probably  one  of  the  most  curious  ex- 
periments in  this  connection  is  that  in  which  a  thin  platinum  wire 
stretched  along  the  axis  of  an  exhausted  tube  is  brought  to  in- 
candescence at  certain  points  corresponding  to  the  position  of 
the  striae,  while  at  others  it  remains  dark.  This  experiment 
throws  an  interesting  light  upon  the  nature  of  the  strife  and  may 
lead  to  important  revelations. 

Mr.  Tesla  also  demonstrated  the  dissipation  of  energy  through 
an  atomic  medium  and  dwelt  upon  the  behavior  of  vacuous 
space  in  conveying  heat,  and  in  this  connection  showed  the  curious 
behavior  of  an  electrode  stream,  from  which  he  concludes  that 
the  molecules  of  a  gas  probably  cannot  be  acted  upon  directly 
at  measurable  distances. 

Mr.  Tesla  summarized  the  chief  results  arrived  at  in  pursuing 
his  investigations  in  a  manner  which  will  serve  as  a  valuable 
guide  to  all  who  may  engage  in  this  work.  Perhaps  most  inter- 
est will  centre  on  his  general  statements  regarding  the  phenomena 
of  phosphorescence,  the  most  important  fact  revealed  in  this  di- 
rection being  that  when  exciting  a  phosphorescent  bulb  a  certain 
definite  potential  gives  the  most  economical  result. 

The  lectures  will  now  be  presented  in  the  order  of  their  date 
of  delivery. 


CHAPTER  XXVI. 

EXPEEIMENTS  WlTH  ALTERNATE  CURRENTS  OF  VERY  HlGH  FRE- 
QUENCY AND  THEIR  APPLICATION  TO  METHODS  OF  ARTIFICIAL 
ILLUMINATION.  J 

THERE  is  no  subject  more  captivating,  more  worthy  of  study, 
than  nature.  To  understand  this  great  mechanism,  to  discover 
the  forces  which  are  active,  and  the  laws  which  govern  them,  is 
the  highest  aim  of  the  intellect  of  man. 

Nature  has  stored  up  in  the  universe  infinite  energy.  The 
eternal  recipient  and  transmitter  of  this  infinite  energy  is  the 
ether.  The  recognition  of  the  existence  of  ether,  and  of  the 
functions  it  performs,  is  one  of  the  most  important  results  of 
modern  scientific  research.  The  mere  abandoning  of  the  idea  of 
action  at  a  distance,  the  assumption  of  a  medium  pervading  all 
space  and  connecting  all  gross  matter,  has  freed  the  minds  of 
thinkers  of  an  ever  present  doubt,  and,  by  opening  a  new  horizon 
— new  and  unforeseen  possibilities — has  given  fresh  interest  to 
phenomena  witli  which  we  are  familiar  of  old.  It  has  been  a 
great  step  towards  the  understanding  of  the  forces  of  nature  and 
their  multifold  manifestations  to  our  senses.  It  has  been  for 
the  enlightened  student  of  physics  what  the  understanding  of 
the  mechanism  of  the  firearm  or  of  the  steam  engine  is  for  the 
barbarian.  Phenomena  upon  which  we  used  to  look  as  wonders 
baffling  explanation,  we  now  see  in  a  different  light.  The  spark 
of  an  induction  coil,  the  glow  of  an  incandescent  lamp,  the  mani- 
festations of  the  mechanical  forces  of  currents  and  magnets  are 
no  longer  beyond  our  grasp ;  instead  of  the  incomprehensible,  as 
before,  their  observation  suggests  now  in  our  minds  a  simple 
mechanism,  and  although  as  to  its  precise  nature  all  is  still  con- 
jecture, yet  we  know  that  the  truth  cannot  be  much  longer  hid- 
den, and  instinctively  we  feel  that  the  understanding  is  dawning 
upon  us.  We  still  admire  these  beautiful  phenomena,  these 

1.  A  lecture  delivered  before  the  American  Institute  of  Electrical  Engineers, 
at  Columbia  College,  N.  Y.,  May  20,  1891. 


146  INVENTIONS  OF  NIKOLA  TESLA. 

strange  forces,  but  we  are  helpless  no  longer ;  we  can  in  a  certain 
measure  explain  them,  account  for  them,  and  we  are  hopeful  of 
finally  succeeding  in  unraveling  the  mystery  which  surrounds 
them. 

Iri  how  far  we  can  understand  the  world  around  us  is  the  ulti- 
mate thought  of  every  student  of  nature.  The  coarseness  of  our 
senses  prevents  us  from  recognizing  the  ulterior  construction  of 
matter,  and  astronomy,  this  grandest  and  most  positive  of  natural 
sciences,  can  only  teach  us  something  that  happens,  as  it  were,  in 
our  immediate  neighborhood ;  of  the  remoter  portions  of  the 
boundless  universe,  with  its  numberless  stars  and  suns,  we  know 
nothing.  But  far  beyond  the  limit  of  perception  of  our  senses 
the  spirit  still  can  guide  us,  and  so  we  may  hope  that  even  these 
unknown  worlds — infinitely  small  and  great — may  in  a  measure 
become  known  to  us.  Still,  even  if  this  knowledge  should  reacli 
us,  the  searching  mind  will  find  a  barrier,  perhaps  forever  unsur- 
passable, to  the  true  recognition  of  that  which  seems  to  be,  the 
mere  appearcmce  of  which  is  the  only  and  slender  basis  of  all 
our  philosophy. 

Of  all  the  forms  of  nature's  immeasurable,  all-pervading 
energy,  which  ever  and  ever  changing  and  moving,  like  a  soul 
animates  the  inert  universe,  electricity  and  magnetism  are  per- 
haps the  most  fascinating.  The  effects  of  gravitation,  of  heat 
and  light  we  observe  daily,  and  soon  we  get  accustomed  to 
them,  and  soon  they  lose  for  us  the  character  of  the  marvelous 
and  wonderful ;  but  electricity  and  magnetism,  with  their  singular 
relationship,  with  their  seemingly  dual  character,  unique -among 
the  forces  in  nature,  with  their  phenomena  of  attractions,  repul- 
sions and  rotations,  strange  manifestations  of  mysterious  agents, 
stimulate  and  excite  the  mind  to  thought  and  research.  "What  is 
electricity,  and  what  is  magnetism  ?  These  questions  have  been 
asked  again  and  again.  The  most  able  intellects  have  ceaselessly 
wrestled  with  the  problem ;  still  the  question  has  not  as  yet  been 
fully  answered.  But  while  we  cannot  even  to-day  state  what 
these  singular  forces  are,  we  have  made  good  headway  to- 
wards the  solution  of  the  problem.  We  are  now  confident  that 
electric  and  magnetic  phenomena  are  attributable  to  ether,  and 
we  are  perhaps  justified  in  saying  that  the  effects  of  static  elec- 
tricity are  effects  of  ether  under  strain,  and  those  of  dynamic 
electricity  and  electro-magnetism  effects  of  ether  in  motion.  But 
this  still  leaves  the  question,  as  to  what  electricity  and  magnetism 
arc,  unanswered. 


HIGH  FREQUENCY  AND  HIGH  POTENTIAL  CURRENTS.      147 

First,  we  naturally  inquire,  What  is  electricity,  and  is  there 
such  a  thing  as  electricity  ?  In  interpreting  electric  phenomena, 
we  may  speak  of  electricity  or  of  an  electric  condition,  state  or 
effect.  If  we  speak  of  electric  effects  we  must  distinguish  two 
such  effects,  opposite  in  character  and  neutralizing  each  other,  as 
observation  shows  that  two  such  opposite  effects  exist.  This  is 
unavoidable,  for  in  a  medium  of  the  properties,  of  ether,  we  can- 
not possibly  exert  a  strain,  or  produce  a  displacement  or  motion 
of  any  kind,  without  causing  in  the  surrounding  medium  an 
equivalent  and  opposite  effect.  But  if  we  speak  of  electricity, 
meaning  a  thing,  we  must,  I  think,  abandon  the  idea  of  two 
electricities,  as  the  existence  of  two  such  things  is  highly  improb- 
able. For  how  can  we  imagine  that  there  should  be  two  things, 
equivalent  in  amount,  alike  in  their  properties,  but  of  opposite 
character,  botli  clinging  to  matter,  both  attracting  and  completely 
neutralizing  each  other?  Such  an  assumption,  though  suggested 
by  many  phenomena,  though  most  convenient  for  explaining 
them,  has  little  to  commend  it.  If  there  is  such  a  thing  as  elec- 
tricity, there  can  be  only  one  such  thing,  and,  excess  and  want 
of  that  one  thing,  possibly;  but  more  probably  its  condition  de- 
termines the  positive  and  negative  character.  The  old  theory  of 
Franklin,  though  falling  short  in  some  respects,  is,  from  a  certain 
point  of  view,  after  all,  the  most  plausible  one.  Still,  in  spite 
of  this,  the  theory  of  the  two  electricities  is  generally  accepted, 
as  it  apparently  explains  electric  phenomena  in  a  more  satisfac- 
tor  manner.  But  a  theory  which  better  explains  the  facts  is  not 
necessarily  true.  Ingenious  minds  will  invent  theories  to  suit 
observation,  and  almost  every  independent  thinker  has  his  own 
views  on  the  subject. 

It  is  not  with  the  object  of  advancing  an  opinion,  but  with 
the  desire  of  acquainting  you  better  with  some  of  the  results, 
which  I  will  describe,  to  show  you  the  reasoning  I  have  fol- 
lowed, the  departures  I  have  made — that  I  venture  to  express, 
in  a  few  words,  the  views  and  convictions  which  have  led  me  to 
these  results. 

I  adhere  to  the  idea  that  there  is  a  thing  which  we  have  been 
in  the  habit  of  calling  electricity.  The  question  is,  What  is  that 
thing?  or,  What,  of  all  tilings,  the  existence  of  which  we  know, 
have  we  the  best  reason  to  call  electricity  \  We  know  that  it  acts 
like  an  incompressible  fluid ;  that  there  must  be  a  constant  quan- 
tity of  it  in  nature ;  that  it  can  be  neither  produced  nor  destroyed ; 


148  INVENTIONS  OF  NIKOLA  TESLA. 

and,  what  is  more  important,  the  electro-magnetic  theory  of  light 
and  all  facts  observed  teach  us  that  electric  and  ether  phenomena 
are  identical.  The  idea  at  once  suggests  itself,  therefore,  that 
electricity  might  be  called  ether.  In  fact,  this  view  has  in  a  cer- 
tain sense  been  advanced  by  Dr.  Lodge.  His  interesting  work 
has  been  read  by  everyone  and  many  have  been  convinced  by 
his  arguments.  His  great  ability  and  the  interesting  nature  of 
the  subject,  keep  the  reader  spellbound ;  but  when  the  impres- 
sions fade,  one  realizes  that  he  has  to  deal  only  with  ingenious 
explanations.  I  must  confess,  that  I  cannot  believe  in  two  elec- 
tricities, much  less  in  a  doubly-constituted  ether.  The  puzzling 
behavior  of  the  ether  as  a  solid  to  waves  of  light  and  heat,  and 
as  a  fluid  to  the  motion  of  bodies  through  it,  is  certainly  ex- 
plained in  the  most  natural  and  satisfactory  manner  by  assuming 
it  to  be  in  motion,  as  Sir  William  Thomson  has  suggested ;  but 
regardless  of  this,  there  is  nothing  which  would  enable  us  to 
conclude  with  certainty  that,  while  a  fluid  is  not  capable  of  trans- 
mitting transverse  vibrations  of  a  few  hundred  or  thousand  per 
second,  it  might  not  be  capable  of  transmitting  such  vibrations 
when  they  range  into  hundreds  of  million  millions  per  second. 
Nor  can  anyone  prove  that  there  are  transverse  ether  waves 
emitted  from  an  alternate  current  machine,  giving  a  small  num- 
ber of  alternations  per  second ;  to  such  slow  disturbances,  the  ether, 
if  at  rest,  may  behave  as  a  true  fluid. 

Returning  to  the  subject,  and  bearing  in  mind  that  the  exist- 
ence of  two  electricities  is,  to  say  the  least,  highly  improbable, 
we  must  remember,  that  we  have  no  evidence  of  electricity,  nor 
can  we  hope  to  get  it,  unless  gross  matter  is  present.  Electricity, 
therefore,  cannot  be  called  ether  in  the  broad  sense  of  the  term ; 
but  nothing  would  seem  to  stand  in  the  way  of  calling  electricity 
ether  associated  with  matter,  or  bound  ether;  or,  in  other  words, 
that  the  so-called  static  charge  of  the  molecule  is  ether  associated 
in  some  way  with  the  molecule.  Looking  at  it  in  that  light,  we 
would  be  justified  in  saying,  that  electricity  is  concerned  in  all 
molecular  actions. 

Now,  precisely  what  the  ether  surrounding  the  molecules  is, 
wherein  it  differs  from  ether  in  general,  can  only  be  conject- 
ured. It  cannot  differ  in  density,  ether  being  incompressible ; 
it  must,  therefore,  be  under  some  strain  or  in  motion,  and  the 
latter  is  the  most  probable.  To  understand  its  functions,  it 
would  be  necessary  to  have  an  exact  idea  of  the  physical  con- 

v  '      • 


HIGH  FREQUENCY  AND  HIGH  POTENTIAL  CURRENTS.      149 

struction  of  matter,  of  which,  of  course,  we  can  only  form  a 
mental  picture. 

But  of  all  the  views  on  nature,  the  one  which  assumes  one 
matter  and  one  force,  and  a  perfect  uniformity  throughout,  is 
the  most  scientific  and  most  likely  to  be  true.  An  infinitesimal 
world,  with  the  molecules  and  their  atoms  spinning  and  moving 
in  orbits,  in  much  the  same  manner  as  celestial  bodies,  carrying 
with  them  and  probably  spinning  with  them  ether,  or  in  other 
words,  carrying  with  them  static  charges,  seems  to  my  mind  the 
most  probable  view,  and  one  which,  in  a  plausible  manner,  ac- 
counts for  most  of  the  phenomena  observed.  The  spinning  of 
the  molecules  and  their  ether  sets  up  the  ether  tensions  or  elec- 
trostatic strains ;  the  equalization  of  ether  tensions  sets  up  ether 
motions  or  electric  currents,  and  the  orbital  movements  produce 
the  effects  of  electro  and  permanent  magnetism. 

About  fifteen  years  ago,  Prof.  Rowland  demonstrated  a  most 
interesting  and  important  fact,  namely,  that  a  static  charge  car- 
ried around  produces  the  effects  of  an  electric  current.  Leaving 
out  of  consideration  the  precise  nature  of  the  mechanism,  which 
produces  the  attraction  and  repulsion  of  currents,  and  conceiving 
the  electrostatically  charged  molecules  in  motion,  this  experimen- 
tal fact  gives'  us  a  fair  idea  of  magnetism.  We  can  conceive  lines 
or  tubes  of  force  which  physically  exist,  being  formed  of  rows 
of  directed  moving  molecules  ;  we  can  see  that  these  lines  must  be 
closed,  that  they  must  tend  to  shorten  and  expand,  etc.  It  like- 
wise explains  in  a  reasonable  way,  the  most  puzzling  phenomenon 
of  all,  permanent  magnetism,  and,  in  general,  lias  all  the  beauties 
of  the  Ampere  theory  without  possessing  the  vital  defect  of  the 
same,  namely,  the  assumption  of  molecular  currents.  Without 
enlarging  further  upon  the  subject,  1  would  say,  that  I  look  upon 
all  electrostatic,  current  and  magnetic  phenomena  as  being  due 
to  electrostatic  molecular  forces. 

The  preceding  remarks  I  have  deemed  necessary  to  a  full 
understanding  of  the  subject  as  it  presents  itself  to  my  mind. 

Of  all  these  phenomena  the  most  important  to  study  are  the 
current  phenomena,  on  account  of  the  already  extensive  and  ever- 
growing use  of  currents  for  industrial  purposes.  It  is  now  a  cen- 
tury since  the  first  practical  source  of  current  was  produced, 
and,  ever  since,  the  phenomena  which  accompany  the  flow  of 
currents  have  been  diligently  studied,  and  through  the  untiring 
efforts  of  scientific  men  the  simple  laws  which  govern  them  have 


150  INVENTIONS  OF  NIKOLA  TESLA. 

been  discovered.  But  these  laws  are  found  to  hold  good  only 
when  the  currents  are  of  a  steady  character.  When  the  currents 
are  rapidly  varying  in  strength,  quite  different  phenomena,  often 
unexpected,  present  themselves,  and  quite  different  laws  hold 
good,  which  even  now  have  not  been  determined  as  fully  as  is 
desirable,  though  through  the  work,  principally,  of  English  scien- 
tists, enough  knowledge  has  been  gained  on  the  subject  to  enable 
us  to  treat  simple  cases  which  now  present  themselves  in  daily 
practice. 

The  phenomena  which  are  peculiar  to  the  changing  character 
of  the  currents  are  greatly  exalted  when  the  rate  of  change  is 
increased,  hence  the  study  of  these  currents  is  considerably  facil- 
itated by  the  employment  of  properly  constructed  apparatus. 
It  was  with  this  and  other  objects  in  view  that  I  constructed 
alternate  current  machines  capable  of  giving  more  than  two 
million  reversals  of  current  per  minute,  and  to  this  circumstance 
it  is  principally  due,  that  I  am  able  to  bring  to  your  attention 
some  of  the  results  thus  far  reached,  which  I  hope  will  prove  to 
be  a  step  in  advance  on  account  of  their  direct  bearing  upon  one 
of  the  most  important  problems,  namely,  the  production  of  a 
practical  and  efficient  source  of  light. 

The  study  of  such  rapidly  alternating  currents  is  very  interest- 
ing. Nearly  every  experiment  discloses  something  new.  Many 
results  may,  of  course,  be  predicted,  but  many  more  are  unfore- 
seen. The  experimenter  makes  many  interesting  observations. 
For  instance,  we  take  a  piece  of  iron  and  hold  it  against  a  magnet. 
Starting  from  low  alternations  and  running  up  higher  and  higher 
we  feel  the  impulses  succeed  each  other  faster  and  faster,  get 
weaker  and  weaker,  and  finally  disappear.  We  then  observe  a 
continuous  pull ;  the  pull,  of  course,  is  not  continuous ;  it  only 
appears  so  to  us ;  our  sense  of  touch  is  imperfect. 

We  may  next  establish  an  arc  between  the  electrodes  and 
observe,  as  the  alternations  rise,  that  the  note  which  accompanies 
alternating  arcs  gets  shriller  and  shriller,  gradually  weakens,  and 
finally  ceases.  The  air  vibrations,  of  course,  continue,  but  they 
are  too  weak  to  be  perceived ;  our  sense  of  hearing  fails  us. 

We  observe  the  small  physiological  effects,  the  rapid  heating  of 
the  iron  cores  and  conductors,  curious  inductive  effects,  interest- 
ing condenser  phenomena,  and  still  more  interesting  light  phe- 
nomena with  a  high  tension  induction  coil.  All  these  experi- 
ments and  observations  would  be  of  the  greatest  interest  to  the 


HIGH  FREQUENCY  AND  HIGH  POTENTIAL  CURRENTS.     151 

student,  but  their  description  would  lead  me  too  far  from  the 
principal  subject.  Partly  for  this  reason,  and  partly  on  account 
of  their  vastly  greater  importance,  I  will  confine  myself  to  the 
description  of  the  light  effects  produced  by  these  currents. 

In  the  experiments  to  this  end  a  high  tension  induction  coil  or 
equivalent  apparatus  for  converting  currents  of  comparatively 
low  into  currents  of  high  tension  is  used. 

If  you  will  be  sufficiently  interested  in  the  results  I  shall  de- 
scribe as  to  enter  into  an  experimental  study  of  this  subject ;  if  you 
will  be  convinced  of  the  truth  of  the  arguments  I  shall  advance — 
your  aim  will  be  to  produce  high  frequencies  and  high  potentials  j 
in  other  words,  powerful  electrostatic  effects.  You  will  then  en- 
counter many  difficulties,  which,  if  completely  overcome,  would 
allow  us  to  produce  truly  wonderful  results. 

First  will  be  met  the  difficulty  of  obtaining  the  required  fre- 
quencies by  means  of  mechanical  apparatus,  and,  if  they  be  ob. 
tained  otherwise,  obstacles  of  a  different  nature  will  present 
themselves.  Next  it  will  be  found  difficult  to  provide  the  requi- 
site insulation  without  considerably  increasing  the  size  of  the 
apparatus,  for  the  potentials  required  are  high,  and,  owing  to  the 
rapidity  of  the  alternations,  the  insulation  presents  peculiar  diffi- 
culties. So,  for  instance,  when  a  gas  is  present,  the  discharge 
may  work,  by  the  molecular  bombardment  of  the  gas  and  con- 
sequent heating,  through  as  much  as  an  inch  of  the  best  solid 
insulating  material,  such  as  glass,  hard  rubber,  porcelain,  sealing 
wax,  etc. ;  in  fact,  through  any  known  insulating  substance.  The 
chief  requisite  in  the  insulation  of  the  apparatus  is,  therefore,  the 
exclusion  of  any  gaseous  matter. 

In  general  my  experience  tends  to  show  that  bodies  which 
possess  the  highest  specific  inductive  capacity,  such  as  glass, 
afford  a  rather  inferior  insulation  to  others,  which,  while  they  are 
good  insulators,  have  a  much  smaller  specific  inductive  capacity, 
such  as  oils,  for  instance,  the  dielectric  losses  being  no  doubt 
greater  in  the  former.  The  difficulty  of  insulating,  of  course, 
only  exists  when  the  potentials  are  excessively  high,  for  with 
potentials  such  as  a  few  thousand  volts  there  is  no  particular  diffi- 
culty encountered  in  conveying  currents  from  a  machine  giving^ 
say,  20,000  alternations  per  second,  to  quite  a  distance.  This 
number  of  alternations,  however,  is  by  far  too  small  for  many 
purposes,  though  quite  sufficient  for  some  practical  applications. 
This  difficulty  of  insulating  is  fortunately  not  a  vital  drawback ; 


153  INVENTIONS  OP  NIKOLA  TESLA. 

it  affects  mostly  the  size  of  the  apparatus,  for,  when  excessively 
high  potentials  would  be  used,  the  light-giving  devices  would  be 
located  not  far  from  the  apparatus,  and  often  they  would  be  quite 
close  to  it.  As  the  air-bombardment  of  the  insulated  wire  is  de- 
pendent on  condenser  action,  the  loss  may  be  reduced  to  a  trifle 
by  using  excessively  thin  wires  heavily  insulated. 

Another  difficulty  will  be  encountered  in  the  capacity  and  self- 
induction  necessarily  possessed  by  the  coil.  If  the  coil  be  large, 
that  is,  if  it  contain  a  great  length  of  wire,  it  will  be  generally 
un  suited  for  excessively  high  frequencies ;  if  it  be  small,  it  may 
be  well  adapted  for  such  frequencies,  but  the  potential  might- 
then  not  be  as  high  as  desired.  A  good  insulator,  and  prefera- 
bly one  possessing  a  small  specific  inductive  capacity,  would 
afford  a  two-fold  advantage.  First,  it  would  enable  us  to  con- 
struct a  very  small  coil  capable  of  withstanding  enormous  differ- 
ences of  potential ;  and  secondly,  such  a  small  coil,  by  reason  of 
its  smaller  capacity  and  self-induction,  would  be  capable  of  a 
quicker  and  more  vigorous  vibration.  The  problem  then  of  con- 
structing a  coil  or  induction  apparatus  of  any  kind  possessing 
the  requisite  qualities  I  regard  as  one  of  no  small  importance, 
and  it  has  occupied  me  for  a  considerable  time. 

The  investigator  who  desires  to  repeat  the  experiments  which 
I  will  describe,  with  an  alternate  current  machine,  capable  of 
supplying  currents  of  the  desired  frequency,  and  an  induction 
coil,  will  do  well  to  take  the  primary  coil  out  and  mount  the  sec- 
ondary in  such  a  manner  as  to  be  able  to  look  through  the  tube 
upon  which  the  secondary  is  wound.  He  will  then  be  able  to 
observe  the  streams  which  pass  from  the  primary  to  the  insulat- 
ing tube,  and  from  their  intensity  he  will  know  jiow  far  he  can 
strain  the  coil.  Without  this  precaution  he  is  sure  to  injure 
the  insulation.  This  arrangment  permito,  however,  an  easy 
exchange  of  the  primaries,  which  is  desirable  in  these  experi- 
ments. 

The  selection  of  the  type  of  machine  best  suited  for  the  pur- 
pose must  be  left  to  the  judgment  of  the  experimenter.  There 
are  here  illustrated  three  distinct  types  of  machines,  which, 
besides  others,  I  have  used  in  my  experiments. 

Fig.  97  represents  the  machine  used  in  my  experiments  before 
this  Institute.  The  field  magnet  consists  of  a  ring  of  wrought 
iron  with  384  pole  projections.  The  armature  comprises  a  steel 
disc  to  which  is  fastened  a  thin,  carefully  welded  rim  of  wrought 


HIGH  FREQUENCY  AND  HIGH  POTENTIAL  CURRENTS.     153 

iron.  Upon  the  rim  are  wound  several  layers  of  fine,  well 
annealed  iron  wire,  which,  when  wound,  is  passed  through 
shellac.  The  armature  wires  are  wound  around  brass  pins, 
wrapped  with  silk  thread.  The  diameter  of  the  armature  wire 
in  this  type  of  machine  should  not  be  more  than  £  of  the  thick- 
ness of  the  pole  projections,  else  the  local  action  will  be  con- 
siderable. 

Fig.  98  represents  a  larger  machine  of  a  different  type.  The 
field  magnet  of  this  machine  consists  of  two  like  parts  which 
either  enclose  an  exciting  coil,  or  else  are  independently  wound. 


FIG.  97. 

Each  part  has  480  pole  projections,  the  projections  of  one  facing 
those  of  the  other.  The  armature  consists  of  a  wheel  of  hard 
bronze,  carrying  the  conductors  which  revolve  between  the  pro- 
jections of  the  field  magnet.  To  wind  the  armature  conductors, 
I  have  found  it  most  convenient  to  proceed  in  the  following 
manner.  I  construct  a  ring  of  hard  bronze  of  the  required  size. 
This  ring  and  the  rim  of  the  wheel  are  provided  with  the 
proper  number  of  pins,  and  both  fastened  upon  a  plate.  The 
armature  conductors  being  wound,  the  pins  are  cut  off  and  the 
ends  of  the  conductors  fastened  by  two  rings  which  screw  to  the 


154 


INVENTIONS  OF  NIKOLA   TE8LA. 


bronze  ring  and  the  rim  of  the  wheel,  respectively.  The  whole 
may  then  be  taken  off  and  forms  a  solid  structure.  The  con- 
ductors in  such  a  type  of  machine  should  consist  of  sheet  copper, 
the  thickness  of  which,  of  course,  depends  on  the  thickness  of 
the  pole  projections;  or  else  twisted  thin  wires  should  be  em- 
ployed. 

Fig.  99  is  a  smaller  machine,  in  many  respects  similar  to  the 
former,  only  here  the  armature  conductors  and  the  exciting  coil 
are  kept  stationary,  while  only  a  block  of  wrought  iron  is  re- 
volved. 

It  would  be  uselessly  lengthening  this  description  were  I  to 


dwell  more  011  the  details  of  construction  of  these  machines. 
Besides,  they  have  been  described  somewhat  more  elaborately  in 
The  Electrical  Engineer,  of  March  18,  1891.  I  deem  it  well, 
however,  to  call  the  attention  of  the  investigator  to  two  things, 
the  importance  of  which,  though  self  evident,  he  is  nevertheless 
apt  to  underestimate ;  namely,  to  the  local  action  in  the  con- 
ductors which  must  be  carefully  avoided,  and  to  the  clearance, 
which  must  be  small.  I  may  add,  that  since  it  is  desirable  to  use 
very  high  peripheral  speeds,  the  armature  should  be  of  very 
large  diameter  in  order  to  avoid  impracticable  belt  speeds.  Of 


SIGH  FREQUENCY  AND  HIGH  POTENTIAL  CURRENTS.     155 

the  several  types  of  these  machines  which  have  been  constructed 
by  me,  I  have  found  that  the  type  illustrated  in  Fig.  97  caused 
me  the  least  trouble  in  construction,  as  well  as  in  maintenance, 
and  on  the  whole,  it  has  been  a  good  experimental  machine. 

In  operating  an  induction  coil  with  very  rapidly  alternating 
currents,  among  the  first  luminous  phenomena  noticed  are  natur- 
ally those  presented  by  the  high-tension  discharge.  As  the  num- 
ber of  alternations  per  second  is  increased,  or  as — the  number 
being  high — the  current  through  the  primary  is  varied,  the  dis- 
charge gradually  changes  in  appearance.  It  would  be  difficult  to 
describe  the  minor  changes  which  occur,  and  the  conditions  which 


i 


FIG.  99. 

bring  them  about,  but  one  may  note  five  distinct  forms  of  the 
discharge. 

First,  one  may  observe  a  weak,  sensitive  discharge  in  the  form 
of  a  thin,  feeble-colored  thread.  (Fig.  lOOa.)  It  always  occurs 
when,  the  number  of  alternations  per  second  being  high,  the  cur- 
rent through  the  primary  is  very  small.  In  spite  of  the  exces- 
sively small  current,  the  rate  of  change  is  great,  and  the  differ- 
ence of  potential  at  the  terminals  of  the  secondary  is  therefore 
considerable,  so  that  the  arc  is  established  at  great  distances ;  but 
the  quantity  of  "  electricity  "  set  in  motion  is  insignificant,  barely 
sufficient  to  maintain  a  thin,  threadlike  arc.  It  is  excessively 
sensitive  and  may  be  made  so  to  such  a  degree  that  the  mere  act 
of  breathing  near  the  coil  will  affect  it,  and  unless  it  is  perfectly 


156  INVENTIONS  OF  NIKOLA  TESLA. 

well  protected  from  currents  of  air,  it  wriggles  around  constantly. 
Nevertheless,  it  is  in  this  form  excessively  persistent,  and  when 
the  terminals  are  approached  to,  say,  one-third  of  the  striking 
distance,  it  can  be  blown  out  only  with  difficulty.  This  excep- 
tional persistency,  when  short,  is  largely  due  to  the  arc  being 
excessively  thin ;  presenting,  therefore,  a  very  small  surface 
to  the  blast.  Its  great  sensitiveness,  when  very  long,  is  probably 
due  to  the  motion  of  the  particles  of  dust  suspended  in  the  air. 

When  the  current  through  the  primary  is  increased,  the  dis- 
charge gets  broader  and  stronger,  and  the  effect  of  the  capacity 
of  the  coil  becomes  visible  until,  finally,  under  proper  conditions, 
a  white  naming  arc,  Fig.  100  B,  often  as  thick  as  one's  finger,  and 
striking  across  the  whole  coil,  is  produced.  It  develops  remark- 
able heat,  and  may  be  further  characterized  by  the  absence  of 
the  high  note  which  accompanies  the  less  powerful  discharges. 
To  take  a  shock  from  the  coil  under  these  conditions  would  not 


FIG.  lOOa.  FIG.  lOOh. 

be  advisable,  although  under  different  conditions,  the  potential 
being  much  higher,  a  shock  from  the  coil  may  be  taken  with 
impunity.  To  produce  this  kind  of  discharge  the  number  of 
alternations  per  second  must  not  be  too  great  for  the  coil  used  ; 
and,  generally  speaking,  certain  relations  between  capacity,  self- 
induction  and  frequency  must  be  observed. 

The  importance  of  these  elements  in  an  alternate  current  cir- 
cuit is  now  well-known,  and  under  ordinary  conditions,  the  gen- 
eral rules  are  applicable.  But  in  an  induction  coil  exceptional 
conditions  prevail.  First,  the  self-induction  is  of  little  importance 
before  the  arc  is  established,  when  it  asserts  itself,  but  perhaps 
never  as  prominently  as  in  ordinary  alternate  current  circuits, 
because  the  capacity  is  distributed  all  along  the  coil,  and  by  reason 
of  the  fact  that  the  coil  usually  discharges  through  very  great 
resistances ;  hence  the  currents  are  exceptionally  small.  Secondly, 


HIGH  FREQUENCY  AND  HIGH  POTENTIAL  CURRENTS.     157 

the  capacity  goes  on  increasing  continually  as  the  potential  rises, 
in  consequence  of  absorption  which  takes  place  to  a  considerable 
extent.  Owing  to  this  there  exists  no  critical  relationship  between 
these  quantities,  and  ordinary  rules  would  not  seem  to  be  appli- 
cable. As  the  potential  is  increased  either  in  consequence  of  the 
increased  frequency  or  of  the  increased  current  through  the 
primary,  the  amount  of  the  energy  stored  becomes  greater  and 
greater,  and  the  capacity  gains  more  and  more  in  importance. 
Up  to  a  certain  point  the  capacity  is  beneficial,  but  after  that  it 
begins  to  be  an  enormous  drawback.  It  follows  from  this  that 
each  coil  gives  the  best  result  with  a  given  frequency  and  primary 
current.  A  very  large  coil,  when  operated  with  currents  of  very 
high  frequency,  may  not  give  as  much  as  •£•  incli  spark.  By  adding 
capacity  to  the  terminals,  the  condition  may  be  improved,  but 
what  the  coil  really  wants  is  a  lower  frequency. 

When  the  flaming  discharge  occurs,  the  conditions  are  evi- 
dently such  that  the  greatest  current  is  made  to  flow  through  the 
circuit.  These  conditions  may  be  attained  by  varying  the  fre- 
quency within  wide  limits,  but  the  highest  frequency  at  which 
the  flaming  arc  can  still  be  produced,  determines,  for  a  given 
primary  current,  the  maximum  striking  distance  of  the  coil.  In 
the  flaming  discharge  the  eclat  effect  of  the  capacity  is  not  per- 
ceptible ;  the  rate  at  which  the  energy  is  being  stored  then  just 
equals  the  rate  at  which  it  can  be  disposed  of  through  the  circuit. 
This  kind  of  discharge  is  the  severest  test  for  a  coil ;  the  break, 
when  it  occurs,  is  of  the  nature  of  that  in  an  overcharged  Ley  den 
jar.  To  give  a  rough  approximation  I  would  state  that,  with  an 
ordinary  coil  of,  say  10,000  ohms  resistance,  the  most  powerful 
arc  would  be  produced  with  about  12,000  alternations  per  second. 

When  the  frequency  is  increased  beyond  that  rate,  the  poten- 
tial, of  course,  rises,  but  the  striking  distance  may,  nevertheless, 
diminish,  paradoxical  as  it  may  seem.  As  the  potential  rises  the 
coil  attains  more  and  more  the  properties  of  a  static  machine 
until,  finally,  one  may  observe  the  beautiful  phenomenon  of  the 
streaming  discharge,  Fig.  101,  which  may  be  produced  across  the 
whole  length  of  the  coil.  At  that  stage  streams  begin  to  issue 
freely  from  all  points  and  projections.  These  streams  will  also  be 
seen  to  pass  in  abundance  in  the  space  between  the  primary  and 
the  insulating  tube.  When  the  potential  is  excessively  high  they 
will  always  appear,  even  if  the  frequency  be  low,  and  even  if  the 
primary  be  surrounded  by  as  much  as  an  inch  of  wax,  hard  nib- 


15g  INVENTIONS  OF  NIKOLA  TE8LA. 

her,  glass,  or  any  other  insulating  substance.  This  limits  greatly 
the' output  of  the  coil,  but  I  will  later  show  how  I  have  been  able 
to  overcome  to  a  considerable  extent  this  disadvantage  in  the 
ordinary  coil. 

Besides  the  potential,  the  intensity  of  the  streams  depends  on 
the  frequency ;  but  if  the  coil  be  very  large  they  show  them- 
selves, no  matter  how  low  the  frequencies  used.  For  instance, 
in  a  very  large  coil  of  a  resistance  of  67,000  ohms,  constructed 
by  me  some  time  ago,  they  appear  with  as  low  as  100  alternations 
per  second  and  less,  the  insulation  of  the  secondary  being  f  inch 
of  ebonite.  When  very  intense  they  produce  a  noise  similar  to 
that  produced  by  the  charging  of  a  Holtz  machine,  but  much 
more  powerful,  and  they  emit  a  strong  smell  of  ozone.  The 
lower  the  frequency,  the  more  apt  they  are  to  suddenly  injure 
the  coil.  With  excessively  high  frequencies  they  may  pass  freely 


FIG.  101. 


without  producing  any  other  effect  than  to  heat  the  insulation 
slowly  and  uniformly. 

The  existence  of  these  streams  shows  the  importance  of  con- 
structing an  expensive  coil  so  as  to  permit  of  one's  seeing 
through  the  tube  surrounding  the  primary,  and  the  latter  should 
be  easily  exchangeable ;  or  else  the  space  between  the  primary 
and  secondary  should  be  completely  filled  up  with  insulating 
material  so  as  to  exclude  all  air.  The  non-observance  of  this 
simple  rule  in  the  construction  of  commercial  coils  is  responsible 
for  the  destruction  of  many  an  expensive  coil. 

At  the  stage  when  the  streaming  discharge  occurs,  or  with 
somewhat  higher  frequencies,  one  may,  by  approaching  the  ter- 
minals quite  nearly,  and  regulating  properly  the  effect  of  capac- 
ity, produce  a  veritable  spray  of  small  silver-white  sparks,  or  a 
bunch  of  excessively  thin  silvery  threads  (Fig.  102)  amidst  a 
powerful  brush — each  spark  or  thread  possibly  corresponding 


HIGH  FREQUENCY  AND  HIGH  POTENTIAL  CURRENTS.     159 

to  one  alternation.  This,  when  produced  under  proper  condi- 
tions, is  probably  the  most  beautiful  discharge,  and  when  an  air 
blast  is  directed  against  it,  it  presents  a  singular  appearance. 
The  spray  of  sparks,  when  received  through  the  body,  causes 
some  inconvenience,  whereas,  when  the  discharge  simply 
streams,  no  tiling  at  all  is  likely  to  be  felt  if  large  conducting 
objects  are  held  in  the  hands  to  protect  them  from  receiving 
small  burns. 

If  the  frequency  is  still  more  increased,  then  the  coil  refuses 
to  give  any  spark  unless  at  comparatively  small  distances,  and  the 
fifth  typical  form  of  discharge  may  be  observed  (Fig.  103).  The 
tendency  to  stream  out  and  dissipate  is  then  so  great  that  when 
the  brush  is  produced  at  one  terminal  no  sparking  occurs,  even 
if,  as  I  have  repeatedly  tried,  the  hand,  or  any  conducting  object, 
is  held  within  the  stream ;  and,  what  is  more  singular,  the  lumi- 


FIG.  103.  FIG.  104. 

nous  stream  is  not  at  all  easily  deflected  by  the  approach  of  a 
conducting  body. 

At  this  stage  the  streams  seemingly  pass  with  the  greatest 
freedom  through  considerable  thicknesses  of  insulators,  and  it  is 
particularly  interesting  to  study  their  behavior.  For  this  pur- 
pose it  is  convenient  to  connect  to  the  terminals  of  the  coil  two 
metallic  spheres  which  may  be  placed  at  any  desired  distance, 
Fig.  104.  Spheres  are  preferable  to  plates,  as  the  discharge  can 
be  better  observed.  By  inserting  dielectric  bodies  between  the 
.spheres,  beautiful  discharge  phenomena  may  be  observed.  If 
the  spheres  be  quite  close  and  a  spark  be  playing  between  them,  by 
interposing  a  thin  plate  of  ebonite  between  the  spheres  the  spark 
instantly  ceases  and  the  discharge  spreads  into  an  intensely  lumi- 
nous circle  several  inches  in  diameter,  provided  the  spheres  are 


160  INVENTIONS  OF  NIKOLA  TE8LA. 

sufficiently  large.  The  passage  of  the  streams  heats,  and,  after 
a  while,  softens,  the  rubber  so  much  that  two  plates  may  be 
made  to  stick  together  in  this  manner.  If  the  spheres  are  so  far 
apart  that  no  spark  occurs,  even  if  they  are  far  beyond  the  strik- 
ing distance,  by  inserting  a  thick  plate  of  glass  the  discharge  is 
instantly  induced  to  pass  from  the  spheres  to  the  glass  in  the 
form  of  luminous  streams.  It  appears  almost  as  though  these 
streams  pass  through  the  dielectric.  In  reality  this  is  not  the 
case,  as  the  streams  are  due  to  the  molecules  of  the  air  which 
are  violently  agitated  in  the  space  between  the  oppositely  charged 
.surfaces  of  the  spheres.  When  no  dielectric  other  than  air  is 
present,  the  bombardment  goes  on,  but  is  too  weak  to  be  visible ; 
by  inserting  a  dielectric  the  inductive  effect  is  much  increased, 
and  besides,  the  projected  air  molecules  find  an  obstacle  and  the 
bombardment  becomes  so  intense  that  the  streams  become  lumi- 
nous. If  by  any  mechanical  means  we  could  effect  such  a  vio- 
lent agitation  of  the  molecules  we  could  produce  the  same  phe- 
nomenon. A  jet  of  air  escaping  through  a  small  hole  under 
enormous  pressure  and  striking  against  an  insulating  substance, 
such  as  glass,  may  be  luminous  in  the  dark,  and  it  might  be  pos- 
sible to  produce  a  phosphorescence  of  the  glass  or  other  insulators 
in  this  manner. 

The  greater  the  specific  inductive  capacity  of  the  interposed 
dielectric,  the  more  powerful  the  effect  produced.  Owing  to 
this,  the  streams  show  themselves  with  excessively  high  poten- 
tials even  if  the  glass  be  as  much  as  one  and  one-half  to  two 
inches  thick.  But  besides  the  heating  due  to  bombardment, 
some  heating  goes  on  undoubtedly  in  th^  dielectric,  being  ap- 
parently greater  in  glass  than  in  ebonite.  I  attribute  this  to  the 
greater  specific  inductive  capacity  of  the  glass,  in  consequence  of 
which,  with  the  same  potential  difference,  a  greater  amount  of 
energy  is  taken  up  in  it  than  in  rubber.  It  is  like  connecting  to 
a  battery  a  copper  and  a  brass  wire  of  the  same  dimensions.  The 
copper  wire,  though  a  more  perfect  conductor,  would  heat  more 
by  reason  of  its  taking  more  current.  Thus  what  is  otherwise 
considered  a  virtue  of  the  glass  is  here  a  defect.  Glass  usually 
gives  way  much  quicker  than  ebonite ;  when  it  is  heated  to  a  cer- 
tain degree,  the  discharge  suddenly  breaks  through  at  one  point, 
assuming  then  the  ordinary  form  of  an  arc. 

The  heating  effect  produced  by  molecular  bombardment  of 
the  dielectric  would,  of  course,  diminish  as  the  pressure  of  the 


HIGH  FREQUENCY  AND  HIGH  POTENTIAL  CURRENTS.     161 

air  is  increased,  and  at  enormous  pressure  it  would  be  negligible, 
unless  the  frequency  would  increase  correspondingly. 

It  will  be  often  observed  in  these  experiments  that  when  the 
spheres  are  beyond  the  striking  distance,  the  approach  of  a  glass 
plate,  for  instance,  may  induce  the  spark  to  jump  between  the 
spheres.  This  occurs  when  the  capacity  of  the  spheres  is  some- 
what below  the  critical  value  which  gives  the  greatest  difference 
of  potential  at  the  terminals  of  the  coil.  By  approaching  a  di- 
electric, the  specific  inductive  capacity  of  the  space  between  the 
spheres  is  increased,  producing  the  same  effect  as  if  the  capacity 
of  the  spheres  were  increased.  The  potential  at  the  terminals 
may  then  rise  so  high  that  the  air  space  is  cracked.  The  experi- 
ment is  best  performed  with  dense  glass  or  mica. 

Another  interesting  observation  is  that  a  plate  of  insulating 
material,  when  the  discharge  is  passing  through  it,  is  strongly 
attracted  by  either  of  the  spheres,  that  is  by  the  nearer  one,  this 
being  obviously  due  to  the  smaller  mechanical  effect  of  the  bom- 
bardment on  that  side,  and  perhaps  also  to  the  greater  electrifica- 
tion. 

From  the  behavior  of  the  dielectrics  in  these  experiments,  we 
may  conclude  that  the  best  insulator  for  these  rapidly  alternating 
currents  would  be  the  one  possessing  the  smallest  specific  induc- 
tive capacity  and  at  the  same  time  one  capable  of  withstanding 
the  greatest  differences  of  potential ;  and  thus  two  diametrically 
opposite  ways  of  securing  the  required  insulation  are  indicated, 
namely,  to  use  either  a  perfect  vacuum  or  a  gas  under  great  press- 
ure ;  but  the  former  would  be  preferable.  Unfortunately  neither 
of  these  two  ways  is  easily  carried  out  in  practice. 

It  is  especially  interesting  to  note  the  behavior  of  an  exces- 
sively high  vacuum  in  these  experiments.  If  a  test  tube,  provided 
with  external  electrodes  and  exhausted  to  the  highest  possible 
degree,  be  connected  to  the  terminals  of  the  coil,  Fig.  105,  the 
electrodes  of  the  tube  are  instantly  brought  to  a  high  temperature 
and  the  glass  at  each  end  of  the  tube  is  rendered  intensely  phos- 
phorescent, but  the  middle  appears  comparatively  dark,  and  for  a 
while  remains  cool. 

When  the  frequency  is  so  high  that  the  discharge  shown  in 
Fig.  103  is  observed,  considerable  dissipation  no  doubt  occurs  in 
the  coil.  Nevertheless  the  coil  may  be  worked  for  a  long  time, 
as  the  heating  is  gradual. 

In  spite  of  the  fact  that  the  difference  of  potential  may  be 


182 


INVENTIONS  OF  NIKOLA  TE8LA. 


enormous,  little  is  felt  when  the  discharge  is  passed  through  the 
body,  provided  the  hands  are  armed.  This  is  to  some  extent  due 
to  the  higher  frequency,  but  principally  to  the  fact  that  less  en- 
ergy is  available  externally,  when  the  difference  of  potential 
reaches  an  enormous  value,  owing  to  the  circumstance  that,  with 
the  rise  of  potential,  the  energy  absorbed  in  the  coil  increases  as 
the  square  of  the  potential.  Up  to  a  certain  point  the  energy 
available  externally  increases  with  the  rise  of  potential,  then  it 
begins  to  fall  off  rapidly.  Thus,  with  the  ordinary  high  tension 
induction  coil,  the  curious  paradox  exists,  that,  while  with  a  given 
current  through  the  primary  the  shock  might  be  fatal,  with  many 
times  that  current  it  might  be  perfectly  harmless,  even  if  the 
frequency  be  the  same.  With  high  frequencies  and  excessively 
high  potentials  when  the  terminals  are  not  connected  to  bodies 
of  some  size,  practically  all  the  energy  supplied  to  the  primary  is 


FIG.  10"). 


FIG.  l<m. 


taken  up  by  the  coil.  There  is  no  breaking  through,  no  local  in- 
jury, but  all  the  material,  insulating  and  conducting,  is  uniformly 
heated. 

To  avoid  misunderstanding  in  regard  to  the  physiological 
effect  of  alternating  currents  of  very  high  frequency,  I  think  it 
necessary  to  state  that,  while  it  is  an  undeniable  fact  that  they  are 
incomparably  less  dangerous  than  currents  of  low  frequencies, 
it  should  not  be  thought  that  they  are  altogether  harmless. 
What  has  just  been  said  refers  only  to  currents  from  an  ordinary 
high  tension  induction  coil,  which  currents  are  necessarily  very 
small ;  if  received  directly  from  a  machine  or  from  a  secondary 
of  low  resistance,  they  produce  more  or  less  powerful  effects,  and 
may  cause  serious  injury,  especially  when  used  in  conjunction 
with  condensers. 


HIGH  FfiEQ  UENOY  AND  HIGH  POTENTIAL  CURRENTS.     163 

The  streaming  discharge  of  a  high  tension  induction  coil  differs 
in  many  respects  from  that  of  a  powerful  static  machine.  In 
color  it  has  neither  the  violet  of  the  positive,  nor  the  brightness 
of  the  negative,  static  discharge,  but  lies  somewhere  between, 
being,  of  course,  alternatively  positive  and  negative.  But  since 
the  streaming  is  more  powerful  when  the  point  or  terminal  is 
electrified  positively,  than  when  electrified  negatively,  it  follows 
that  the  point  of  the  brush  is  more  like  the  positive,  and  the  root 
more  like  the  negative,  static  discharge.  In  the  dark,  when  the 
brush  is  very  powerful,  the  root  may  appear  almost  white.  The 
wind  produced  by  the  escaping  streams,  though  it  may  be  very 
strong — often  indeed  to  such  a  degree  that  it  may  be  felt  quite  a 
distance  from  the  coil — is,  nevertheless,  considering  the  quantity 
of  the  discharge,  smaller  than  that  produced  by  the  positive 


FIG.  107.  FIG.  108. 

brush  of  a  static  machine,  and  it  affects  the  flame  much  less 
powerfully.  From  the  nature  of  the  phenomenon  we  can  con- 
clude that  the  higher  the  frequency,  the  smaller  must,  of  course, 
be  the  wind  produced  by  the  streams,  and  with  sufficiently  high 
frequencies  no  wind  at  all  would  be  produced  at  the  ordinary 
atmospheric  pressures.  With  frequencies  obtainable  by  means 
of  a  machine,  the  mechanical  effect  is  sufficiently  great  to  revolve, 
with  considerable  speed,  large  pin-wheels,  which  in  the  dark 
present  a  beautiful  appearance  owing  to  the  abundance  of  the 
streams  (Fig.  106). 

In  general,  most  of  the  experiments  usually  performed  with  a 
static  machine  can  be  performed  with  an  induction  coil  when 
operated  witli  very  rapidly  alternating  currents.  The  effects  pro- 
duced, however,' are  much  more  striking,  being  of  incomparably 


164  INVENTIONS  OF  NIKOLA  TESLA. 

greater  power.  When  a  small  length  of  ordinary  cotton  covered 
wire,  Fig.  107,  is  attached  to  one  terminal  of  the  coil,  the  streams 
issuing  from  all  points  of  the  wire  may  be  so  intense  as  to  produce 
a  considerable  light  effect.  When  the  potentials  and  frequencies 
are  very  high,  a  wire  insulated  with  gutta  percha  or  rubber  and 
attached  to  one  of  the  terminals,  appears  to  be  covered  with  a 
luminous  film.  A  very  thin  bare  wire  when  attached  to  a  ter- 
minal emits  powerful  streams  and  vibrates  continually  to  and  fro 
or  spins  in  a  circle,  producing  a  singular  effect  (Fig.  108).  Some 
of  these  experiments  have  been  described  by  me  in  The  Electrical 
W(M,  of  February  21,  1891. 

Another  peculiarity  of  the  rapidly  alternating  discharge  of  the 
induction  coil  is  its  radically  different  behavior  with  respect  to 
points  and  rounded  surfaces. 

If  a  thick  wire,  provided  with  a  ball  at  one  end  and  with  a 
point  at  the  other,  be  attached  to  the  positive  terminal  of  a  static 
machine,  practically  all  the  charge  will  be  lost  through  the  point, 
on  account  of  the  enormously  greater  tension,  dependent  on  the 
radius  of  curvature.  But  if  such  a  wire  is  attached  to  one  of  the 
terminals  of  the  induction  coil,  it  will  be  observed  that  with  very 
high  frequencies  streams  issue  from  the  ball  almost  as  copiously 
as  from  the  point  (Fig.  109). 

It  is  hardly  conceivable  that  we  could  produce  such  a  condi- 
tion to  an  equal  degree  in  a  static  machine,  for  the  simple  reason, 
that  the  tension  increases  as  the  square  of  the  density,  which  in 
turn  is  proportional  to  the  radius  of  curvature ;  hence,  with  a 
steady  potential  an  enormous  charge  would  be  required  to  make 
streams  issue  from  a  polished  ball  while  it  is  connected  with  a 
point.  But  with  an  induction  coil  the  discharge  of  which  alter- 
nates with  great  rapidity  it  is  different,  Here  we  have  to  deal 
with  two  distinct  tendencies.  First,  there  is  the  tendency  to 
escape  which  exists  in  a  condition  of  rest,  and  which  depends  OH 
the  radius  of  curvature;  second,  there  is  the  tendency  to  dissi- 
pate into  the  surrounding  air  by  condenser  action,  which  de- 
pends on  the  surface.  When  one  of  these  tendencies  is  a  maxi- 
mum, the  other  is  at  a  minimum.  At  the  point  the  luminous 
stream  is  principally  due  to  the  air  molecules  coming  bodily  in 
contact  with  the  point ;  they  are  attracted  and  repelled,  charged 
and  discharged,  and,  their  atomic  charges  being  thus  disturbed, 
vibrate  and  emit  light  waves.  At  the  ball,  on  the  contrary,  there 
is  no  doubt  that  the  effect  is  to  a  great  extent  produced  indue- 


&IGH  FREQUENCY  AND  HIGH  POTENTIAL  CURRENTS.     166 

tively,  the  air  molecules  not  necessarily  coining  in  contact  with 
the  ball,  though  they  undoubtedly  do  so.  To  convince  ourselves 
of  this  we  only  need  to  exalt  the  condenser  action,  for  instance, 
by  enveloping  the  ball,  at  some  distance,  by  a  better  conductor 
than  the  surrounding  medium,  the  conductor  being,  of  course, 
insulated ;  or  else  by  surrounding  it  with  a  better  dielectric  and 
approaching  an  insulated  conductor;  in  both  cases  the  streams 
will  break  forth  more  copiously.  Also,  the  larger  the  ball  with 
a  given  frequency,  or  the  higher  the  frequency,  the  more  will 
the  ball  have  the  advantage  over  the  point.  But,  since  a  certain 
intensity  of  action  is  required  to  render  the  streams  visible,  it  is 
obvious  that  in  the  experiment  described  the  ball  should  not  be 
taken  too  large. 

In  consequence  of  this  two-fold  tendency,  it  is  possible  to  pro- 
duce by  means  of  points,  effects  identical   to  those  produced  by 


FIG.  109.  FIG.  110. 

capacity.  Thus,  for  instance,  by  attaching  to  one  terminal  of 
the  coil  a  small  length  of  soiled  wire,  presenting  many  points 
and  offering  great  facility  to  escape,  the  potential  of  the  coil 
may  be  raised  to  the  same  value  as  by  attaching  to  the  terminal 
a  polished  ball  of  a  surface  many  times  greater  than  that  of  the 
wire. 

An  interesting  experiment,  showing  the  effect  of  the  points, 
may  be  performed  in  the  following  manner :  Attach  to  one  of 
the  terminals  of  the  coil  a  cotton  covered  wire  about  two  feet  in 
length,  and  adjust  the  conditions  so  that  streams  issue  from  the 
wire.  In  this  experiment  the  primary  coil  should  be  preferably 
placed  so  that  it  extends  only  about  half  way  into  the  secondary 
coil.  Now  touch  the  free  terminal  of  the  secondary  with  a  con- 
ducting object  held  in  the  hand,  or  else  connect  it  to  an  insulated 


166  INVENTIONS  OF  NIKOLA  TESLA. 

body  of  some  size.  In  this  manner  the  potential  on  the  wire 
may  be  enormously  raised.  The  effect  of  this  will  be  either  to 
increase,  or  to  diminish,  the  streams.  If  they  increase,  the  wire 
is  too  short ;  if  they  diminish,  it  is  too  long.  By  adjusting  the 
length  of  the  wire,  a  point  is  found  where  the  touching  of  the 
other  terminal  does  not  at  all  affect  the  streams.  In  this  case 
the  rise  of  potential  is  exactly  counteracted  by  the  drop  through 
the  coil.  It  will  be  observed  that  small  lengths  of  wire  produce 
considerable  difference  in  the  magnitude  and  luminosity  of  the 
streams.  The  primary  coil  is  placed  side  wise  for  two  reasons: 
First,  to  increase  the  potential  at  the  wire ;  and,  second,  to  in- 
crease the  drop  through  the  coil.  The  sensitiveness  is  thus  aug- 
mented. 

There  is  still  another  and  far  more  striking  peculiarity  of  the 
brush  discharge  produced  by  very  rapidly  alternating  currents. 
To  observe  this  it  is  best  to  replace  the  usual  terminals  of  the 
coil  by  two  metal  columns  insulated  with  a  good  thickness  of 
ebonite.  It  is  also  well  to  close  all  fissures  and  cracks  with  wax 
so  that  the  brushes  cannot  form  anywhere  except  at  the  tops  of 
the  columns.  If  the  conditions  are  carefully  adjusted — which, 
of  course,  must  be  left  to  the  skill  of  the  experimenter — so  that 
the  potential  rises  to  an  enormous  value,  one  may  produce  two 
powerful  brushes  several  inches  long,  nearly  white  at  their  roots, 
which  in  the  dark  bear  a  striking  resemblance  to  two  flames  of 
a  gas  escaping  under  pressure  (Fig.  110).  But  they  do  not  only 
resemble,  they  are  veritable  flames,  for  they  are  hot.  Certainly 
they  are  not  as  hot  as  a  gas  burner,  but  they  would  be  so  if  the 
frequency  cmd  the  potential  would  be  sufficiently  high.  Produced 
with,  say,  twenty  thousand  alternations  per  second,  the  heat  is 
easily  perceptible  even  if  the  potential  is  not  excessively  high. 
The  heat  developed  is,  of  course,  due  to  the  impact  of  the  air 
molecules  against  the  terminals  and  against  each  other.  As,  at 
the  ordinary  pressures,  the  mean  free  path  is  excessively  small, 
it  is  possible  that  in  spite  of  the  enormous  initial  speed  imparted 
to  each  molecule  upon  coming  in  contact  with  the  terminal,  its 
progress — by  collision  with  other  molecules — is  retarded  to  such 
an  extent,  that  it  does  not  get  away  far  from  the  terminal,  but 
may  strike  the  same  many  times  in  succession.  The  higher  the 
frequency,  the  less  the  molecule  is  able  to  get  away,  and  this  the 
more  so,  as  for  a  given  effect  the  potential  required  is  smaller ; 
and  a  frequency  is  conceivable — perhaps  even  obtainable — at 


HIGH  FREQUENCY  AND  HIGH  POTENTIAL  CURRENTS.     167 

which  practically  the  same  molecules  would  strike  the  terminal. 
Under  such  conditions  the  exchange  of  the  molecules  would  be 
very  slow,  and  the  heat  produced  at,  and  very  near,  the  terminal 
would  be  excessive.  But  if  the  frequency  would  go  on  increasing 
constantly,  the  heat  produced  would  begin  to  diminish  for  ob- 
vious reasons.  In  the  positive  brush  of  a  static  machine  the  ex- 
change of  the  molecules  is  very  rapid,  the  stream  is  constantly 
of  one  direction,  and  there  are  fewer  collisions ;  hence  the  heating 
effect  must  be  very  small.  Anything  that  impairs  the  facility 
of  exchange  tends  to  increase  the  local  heat  produced.  Thus,  if 
a  bulb  be  held  over  the  terminal  of  the  coil  so  as  to  enclose  the 
brush,  the  air  contained  in  the  bulb  is  very  quickly  brought  to 
a  high  temperature.  If  a  glass  tube  be  held  over  the  brush  so 
as  to  allow  the  draught  to  carry  the  brush  upwards,  scorching  hot 
air  escapes  at  the  top  of  the  tube.  Anything  held  within  the 
brush  is,  of  course,  rapidly  heated,  and  the  possibility  of  using 
such  heating  effects  for  some  purpose  or  other  suggests  itself. 

When  contemplating  this  singular  phenomenon  of  the  hot 
brush,  we  cannot  help  being  convinced  that  a  similar  process 
must  take  place  in  the  ordinary  flame,  and  it  seems  strange  that 
after  all  these  centuries  past  of  familiarity  with  the  flame,  now, 
in  this  era  of  electric  lighting  and  heating,  we  are  finally  led  to 
recognize,  that  since  time  immemorial  we  have,  after  all,  always 
had  "  electric  light  and  heat "  at  our  disposal.  It  is  also  of  no 
little  interest  to  contemplate,  that  we  have  a  possible  way  of 
producing — by  other  than  chemical  means — a  veritable  flame, 
which  would  give  light  and  heat  without  any  material  being 
consumed,  without  any  chemical  process  taking  place,  and  to 
accomplish  this,  we  only  need  to  perfect  methods  of  producing 
enormous  frequencies  and  potentials.  I  have  no  doubt  that  if 
the  potential  could  be  made  to  alternate  with  sufficient  rapidity 
and  power,  the  brush  formed  at  the  end  of  a  wire  would  lose  its 
electrical  characteristics  and  would  become  flamelike.  The  flame 
must  be  due  to  electrostatic  molecular  action. 

This  phenomenon  now  explains  in  a  manner  which  can  hardly 
be  doubted  the  frequent  accidents  occurring  in  storms.  It  is  well 
known  that  objects  are  often  set  on  fire  without  the  lightning 
striking  them.  We  shall  presently  see  how  this  can  happen. 
On  a  nail  in  a  roof,  for  instance,  or  on  a  projection  of  any  kind, 
more  or  less  conducting,  or  rendered  so  by  dampness,  a  powerful 
brush  may  appear.  If  the  lightning  strikes  somewhere  in  the 


168  INVENTIONS  OF  NIKOLA  TESLA. 

neighborhood  the  enormous  potential  may  be  made  to  alternate 
or  fluctuate  perhaps  many  million  times  a  second.  The  air 
molecules  are  violently  attracted  and  repelled,  and  by  their  im- 
pact produce  such  a  powerful  heating  effect  that  a  lire  is  started. 
It  is  conceivable  that  a  ship  at  sea  may,  in  this  manner,  catch  fire 
at  many  points  at  once.  When  we  consider,  that  even  with  the 
comparatively  low  frequencies  obtained  from  a  dynamo  machine, 
and  with  potentials  of  no  more  than  one  or  two  hundred  thous- 
and volts,  the  heating  effects  are  considerable,  we  may  imagine 
how  much  more  powerful  they  must  be  with  frequencies  and  po- 
tentials many  times  greater;  and  the  above  explanation  seems,  to 
say  the  least,  very  probable.  Similar  explanations  may  have  been 
suggested,  but  I  am  not  aware  that,  up  to  the  present,  the  heat- 
ing effects  of  a  brush  produced  by  a  rapidly  alternating  potential 


FIG.  ill. 

have  been  experimentally  demonstrated,  at  least  not  to  such  a 
remarkable  degree. 

By  preventing  completely  the  exchange  of  the  air  molecules^ 
the  local  heating  effect  may  be  so  exalted  as  to  bring  a  body  to 
incandescence.  Thus,  for  instance,  if  a  small  button,  or  prefer- 
ably a  very  thin  wire  or  filament  be  enclosed  in  an  unexhausted 
globe  and  connected  with  the  terminal  of  the  coil,  it  may  be 
rendered  incandescent.  The  phenomenon  is  made  much  more 
interesting  by  the  rapid  spinning  round  in  a  circle  of  the  top  of 
the  filament,  thus  presenting  the  appearance  of  a  luminous  fun- 
nel, Fig.  Ill,  which  widens  when  the  potential  is  increased. 
When  the  potential  is  small  the  end  of  the  filament  may  perf  orm 
irregular  motions,  suddenly  changing  from  one  to  the  other,  or 
it  may  describe  an  ellipse;  but  when  the  potential  is  very 
high  it  always  spins  in  a  circle ;  and  so  does  generally  a  thin 


HIGH  FREQUENCY  AND  HIGH  POTENTIAL  CURRENTS.     169 

straight  wire  attached  freely  to  the  terminal  of  the  coil.  These 
motions  are,  of  course,  due  to  the  impact  of  the  molecules,  and 
the  irregularity  in  the  distribution  of  the  potential,  owing  to  the 
roughness  and  dissymmetry  of  the  wire  or  filament.  With  a 
perfectly  symmetrical  and  polished  wire  such  motions  would 
probably  not  occur.  That  the  motion  is  not  likely  to  be  due  to 
others  causes  is  evident  from  the  fact  that  it  is  not  of  a  definite 
direction,  and  that  in  a  very  highly  exhausted  globe  it  ceases 
altogether.  The  possibility  of  bringing  a  body  to  incandescence 
in  an  exhausted  globe,  or  even  when  not  at  all  enclosed,  would 
seem  to  afford  a  possible  way  of  obtaining  light  eifects,  which, 
in  perfecting  methods  of  producing  rapidly  alternating  potentials, 
might  be  rendered  available  for  useful  purposes. 

In  employing  a  commercial  coil,  the  production  of  very  power- 
ful brush  effects   is  attended  with  considerable  difficulties,  for 


FIG.  112*. 

when  these  high  frequencies  and  enormous  potentials  are  used, 
the  best  insulation  is  apt  to  give  way.  Usually  the  coil  is  insu- 
lated well  enough  to  stand  the  strain  from  convolution  to  convo- 
lution, since  two  double  silk  covered  paraffined  wires  will  with- 
stand a  pressure  of  several  thousand  volts;  the  difficulty  lies 
principally  in  preventing  the  breaking  through  from  the  secon- 
dary to  the  primary,  which  is  greatly  facilitated  by  the  streams 
issuing  from  the  latter.  In  the  coil,  of  course,  the  strain  is  great- 
est from  section  to  section,  but  usually  in  a  larger  coil  there  are 
so  many  sections  that  the  danger  of  a  sudden  giving  way  is  not 
very  great.  No  difficulty  will  generally  be  encountered  in  that 
direction,  and  besides,  the  liability  of  injuring  the  coil  internally 
is  very  much  reduced  by  the  fact  that  the  effect  most  likely  to 
be  produced  is  simply  a  gradual  heating,  which,  when  far  enough 


1?0  INVENTIONS  OF  NIKOLA  TE8LA. 

advanced,  could  not  fail  to  be  observed.  The  principal  necessity 
is  then  to  prevent  the  streams  between  the  primary  and  the  tube, 
not  only  on  account  of  the  heating  and  possible  injury,  but  also 
because  the  streams  may  diminish  very  considerably  the  potential 
difference  available  at  the  terminals.  A  few  hints  as  to  how 
this  may  be  accomplished  will  probably  be  found  useful  in  most 
of  these  experiments  with  the  ordinary  induction  coil. 

One  of  the  ways  is  to  wind  a  short  primary,  Fig.  112a,  so  that 
the  difference  of  potential  is  not  at  that  length  great  enough  to 
cause  the  breaking  forth  of  the  streams  through  the  insulating 
tube.  The  length  of  the  primary  should  be  determined  by  expe- 
riment. Both  the  ends  of  the  coil  should  be  brought  out  on  one 
end  through  a  plug  of  insulating  material  fitting  in  the  tube  as 
illustrated.  In  such  a  disposition  one  terminal  of  the  secondary 
is  attached  to  a  body,  the  surface  of  which  is  determined  with  the 


FIG.  112b. 

greatest  care  so  as  to  produce  the  greatest  rise  in  the  potential. 
At  the  other  terminal  a  powerful  brush  appears,  which  may  be 
experimented  upon. 

The  above  plan  necessitates  the  employment  of  a  primary  of 
comparatively  small  size,  and  it  is  apt  to  heat  when  powerful  ef- 
fects are  desirable  for  a  certain  length  of  time.  In  such  a  case  it 
is  better  to  employ  a  larger  coil,  Fig.  112b,  and  introduce  it 
from  one  side  of  the  tube,  until  the  streams  begin  to  appear.  In 
this  case  the  nearest  terminal  of  the  secondary  may  be  connected 
to  the  primary  or  to  the  ground,  which  is  practically  the  same 
thing,  if  the  primary  is  connected  directly  to  the  machine.  In  the 
case  of  ground  connections  it  is  well  to  determine  experimentally 
the  frequency  which  is  best  suited  under  the  conditions  of  the 
test.  Another  way  of  obviating  the  streams,  more  or  less,  is  to 


Iimil  FRKQURNOY  AND  HIGH  POTENTIAL  CURRENTS.     Ill 

make  the  primary  in  sections  and  supply  it  from  separate,  well 
insulated  sources. 

In  many  of  these  experiments,  when  powerful  effects  are 
wanted  for  a  short  time,  it  is  advantageous  to  use  iron  cores  with 
the  primaries.  In  such  case  a  very  large  primary  coil  may  be 
wound  and  placed  side  by  side  witli  the  secondary,  and,  the  near- 
est terminal  of  the  latter  being  connected  to  the  primary,  a  lami- 
nated iron  core  is  introduced  through  the  primary  into  the  sec- 
ondary as  far  as  the  streams  will  permit.  Under  these  conditions 
an  excessively  powerful  brush,  several  inches  long,  which  may 
be  appropriately  called  "  St.  Elmo's  hot  fire,"  may  be  caused  to 
appear  at  the  other  terminal  of  the  secondary,  producing  striking 
effects.  It  is  a  most  powerful  ozonizer,  so  powerful  indeed,  that 
only  a  few  minutes  are  sufficient  to  fill  the  whole  room  with  the 
smell  of  ozone,  and  it  undoubtedly  possesses  the  quality  of  excit- 
ing chemical  affinities. 

For  the  production  of  ozone,  alternating  currents  of  very 
high  frequency  are  eminently  suited,  not  only  on  account  of  the 
advantages  they  offer  in  the  way  of  conversion  but  also  because 
of  the  fact,  that  the  ozonizing  action  of  a  discharge  is  dependent 
on  the  frequency  as  well  as  on  the  potential,  this  being  undoubt- 
edly confirmed  by  observation. 

In  these  experiments  if  an  iron  core  is  used  it  should  be  care- 
fully watched,  as  it  is  apt  to  get  excessively  hot  in  an  incredibly 
short  time.  To  give  an  idea  of  the  rapidity  of  the  heating,  I 
will  state,  that  by  passing  a  powerful  current  through  a  coil  with 
many  turns,  the  inserting  within  the  same  of  a  thin  iron  wire  for 
no  more  than  one  second's  time  is  sufficient  to  heat  the  wire  to 
something  like  100°  C. 

But  this  rapid  heating  need  not  discourage  us  in  the  use 
of  iron  cores  in  connection  with  rapidly  alternating  currents. 
I  have  for  a  long  time  been  convinced  that  in  the  industrial  distri- 
bution by  means  of  transformers,  some  such  plan  as  the  following 
might  be  practicable.  We  may  use  a  comparatively  small  iron 
core,  subdivided,  or  perhaps  not  even  subdivided.  We  may  sur- 
round this  core  with  a  considerable  thickness  of  material  which 
is  fire-proof  and  conducts  the  heat  poorly,  and  on  top  of  that  we 
may  place  the  primary  and  secondary  windings.  By  using  either 
higher  frequencies  or  greater  magnetizing  forces,  we  may  by 
hysteresis  and  eddy  currents  heat  the  iron  core  so  far  as  to  bring 
it  nearly  to  its  maximum  permeability,  which,  as  Hopkinson  has 


172  INVENTION'S  OF  NIKOLA  TE8LA. 

shown,  may  be  as  much  as  sixteen  times  greater  than  that  at  or- 
dinary temperatures.  If  the  iron  core  were  perfectly  enclosed, 
it  would  not  be  deteriorated  by  the  heat,  and,  if  the  enclosure  of 
tire-proof  material  would  be  sufficiently  thick,  only  a  limited 
amount  of  energy  could  be  radiated  in  spite  of  the  high  tem- 
perature. Transformers  have  been  constructed  by  me  on  that 
plan,  but  for  lack  of  time,  no  thorough  tests  have  as  yet  been 
made. 

Another  way  of  adapting  the  iron  core  to  rapid  alternations, 
or,  generally  speaking,  reducing  the  frictional  losses,  is  to  pro- 
duce by  continuous  magnetization  a  flow  of  something  like  seven 
thousand  or  eight  thousand  lines  per  square  centimetre  through 
the  core,  and  then  work  with  weak  magnetizing  forces  and  pre- 
ferably high  frequencies  around  the  point  of  greatest  permeabil- 
ity. A  higher  efficiency  of  conversion  and  greater  output  are 
obtainable  in  this  manner.  I  have  also  employed  this  principle 
in  connection  with  machines  in  which  there  is  no  reversal  of 
polarity.  In  these  types  of  machines,  as  long  as  there  are  only 
few  pole  projections,  there  is  no  great  gain,  as  the  maxima  and 
minima  of  magnetization  are  far  from  the  point  of  maximum 
permeability ;  but  when  the  number  of  the  pole  projections  is 
very  great,  the  required  rate  of  change  may  be  obtained,  without 
the  magnetization  varying  so  far  as  to  depart  greatly  from  the 
point  of  maximum  permeability,  and  the  gain  is  considerable. 

The  above  described  arrangements  refer  only  to  the  use  of 
commercial  coils  as  ordinarily  constructed.  If  it  is  desired  to 
construct  a  coil  for  the  express  purpose  of  performing  with  it 
such  experiments  as  I  have  described,  or,  generally,  rendering  it 
capable  of  withstanding  the  greatest  possible  difference  of  poten- 
tial, then  a  construction  as  indicated  in  Fig.  113  will  be  found  of 
advantage.  The  coil  in  this  case  is  formed  of  two  independent 
parts  which  are  wound  oppositely,  the  connection  between  both 
being  made  near  the  primary.  The  potential  in  the  middle  being 
zero,  there  is  not  much  tendency  to  jump  to  the  primary  and  not 
much  insulation  is  required.  In  some  cases  the  middle  point 
may,  however,  be  connected  to  the  primary  or  to  the  ground.  In 
such  a  coil  the  places  of  greatest  difference  of  potential  are  far 
apart  and  the  coil  is  capable  of  withstanding  an  enormous  strain. 
The  two  parts  may  be  movable  so  as  to  allow  a  slight  adjustment 
of  the  capacity  effect. 

As  to  the  manner  of  insulating  the  coil,  it  will  be  found  con- 


HIGH  FREQUENCY  AND  HIGH  POTENTIAL  CURRENTS.     173 

venient  to  proceed  in  the  following  way :  First,  the  wire  should 
be  boiled  in  paraffine  until  all  the  air  is  out ;  then  the  coil  is 
wound  by  running  the  wire  through  melted  paraffine,  merely  for 
the  purpose  of  fixing  the  wire.  The  coil  is  then  taken  off  from 
the  spool,  immersed  in  a  cylindrical  vessel  filled  with  pure  melted 
wax  and  boiled  for  a  long  time  until  the  bubbles  cease  to  appear. 
The  whole  is  then  left  to  cool  down  thoroughly,  and  then  the 
mass  is  taken  out  of  the  vessel  and  turned  up  in  a  lathe.  A  coil 
made  in  this  manner  and  with  care  is  capable  of  withstanding 
enormous  potential  differences. 

It  may  be  found  convenient  to  immerse  the  coil  in  paraffine  oil 
or  some  other  kind  of  oil ;  it  is  a  most  effective  way  of  insulating, 
principally  on  account  of  the  perfect  exclusion  of  air,  but  it  may 


FIG.  113. 


be  found  that,  after  all,  a  vessel  filled  with  oil  is  not  a  very  con- 
venient thing  to  handle  in  a  laboratory. 

If  an  ordinary  coil  can  be  dismounted,  the  primary  may  be 
taken  out  of  the  tube  and  the  latter  plugged  up  at  one  end,  filled 
with  oil,  and  the  primary  reinserted.  This  affords  an  excellent 
insulation  and  prevents  the  formation  of  the  streams. 

Of  all  the  experiments  which  may  be  performed  with  rapidly 
alternating  currents  the  most  interesting  are  those  which  concern 
the  production  of  a  practical  illuminant.  It  cannot  be  denied 
that  the  present  methods,  though  they  were  brilliant  advances, 
are  very  wasteful.  Some  better  methods  must  be  invented,  some 
more  perfect  apparatus  devised.  Modern  research  has  opened 
new  possibilities  for  the  production  of  an  efficient  source  of  light, 
and  the  attention  of  all  has  been  turned  in  the  direction  indicated 


174  INVENTIONS  OF  NIKOLA  TESLA. 

by  able  pioneers.  Many  have  been  carried  away  by  the  enthusiasm 
and  passion  to  discover,  but  in  their  zeal  to  reach  results,  some 
have  been  misled.  Starting  with  the  idea  of  producing  electro- 
magnetic waves,  they  turned  their  attention,  perhaps,  too  much 
to  the  study  of  electro-magnetic  effects,  and  neglected  the  study 
of  electrostatic  phenomena.  Naturally,  nearly  every  investigator 
availed  himself  of  an  apparatus  similar  to  that  used  in  earlier 
experiments.  But  in  those  forms  of  apparatus,  while  the  electro- 
magnetic inductive  effects  are  enormous,  the  electrostatic  effects 
are  excessively  small. 

In  the  Hertz  experiments,  for  instance,  a  high  tension  induc- 
tion coil  is  short  circuited  by  an  arc,  the  resistance  of  which  is 
very  small,  the  smaller,  the  more  capacity  is  attached  to  the  ter- 
minals ;  and  the  difference  of  potential  at  these  is  enormously 
diminished.  On  the  other  hand,  when  the  discharge  is  not  pass- 
ing between  the  terminals,  the  static  effects  may  be  considerable, 
but  only  qualitatively  so,  not  quantitatively,  since  their  rise  and 
fall  is  very  sudden,  and  since  their  frequency  is  small.  In  neither 
case,  therefore,  are  powerful  electrostatic  effects  perceivable. 
Similar  conditions  exist  when,  as  in  some  interesting  experiments 
of  Dr.  Lodge,  Ley  den  jars  are  discharged  disruptively.  It  has 
been  thought  —  and  I  believe  asserted  —  that  in  such  cases 
most  of  the  energy  is  radiated  into  space.  In  the  light  of  the 
experiments  which  I  have  described  above,  it  will  now  not  be 
thought  so.  I  feel  safe  in  asserting  that  in  such  cases  'most  of 
the  energy  is  partly  taken  up  and  converted  into  heat  in  the  arc 
of  the  discharge  and  in  the  conducting  and  insulating  material  of 
the  jar,  some  energy  being,  of  course,  given  off  by  electrification 
of  the  air ;  but  the  amount  of  the  directly  radiated  energy  is  very 
small. 

When  a  high  tension  induction  coil,  operated  by  currents  alter- 
nating only  20,000  times  a  second,  has  its  terminals  closed  through 
even  a  very  small  jar,  practically  all  the  energy  passes  through 
the  dielectric  of  the  jar,  which  is  heated,  and  the  electrostatic 
effects  manifest  themselves  outwardly  only  to  a  very  weak  degree. 
Now  the  external  circuit  of  a  Leyden  jar,  that  is,  the  arc  and  the 
connections  of  the  coatings,  may  be  looked  upon  as  a  circuit  gen- 
erating alternating  currents  of  excessively  high  frequency  and 
fairly  high  potential,  which  is  closed  through  the  coatings  and 
the  dielectric  between  them,  and  from  the  above  it  is  evident 
that  the  external  electrostatic  effects  must  be  very  small,  even  if  a 


HIGH  FREQUENCY  AND  HIGH  POTENTIAL  CURRENTS.      175 

recoil  circuit  be  used.  These  conditions  make  it  appear  that  with 
the  apparatus  usually  at  hand,  the  observation  of  powerful  elec- 
trostatic effects  was  impossible,  and  what  experience  has  been 
gained  in  that  direction  is  only  due  to  the  great  ability  of  the 
investigators. 

But  powerful  electrostatic  eifects  are  a  sitw  qua  -non  of  light 
production  on  the  lines  indicated  by  theory.  Electro-magnetic 
eifects  are  primarily  unavailable,  for  the  reason  that  to  produce 
the  required  effects  we  would  have  to  pass  current  impulses 
through  a  conductor,  which,  long  before  the  required  frequency 
of  the  impulses  could  be  reached,  would  cease  to  transmit  them. 
On  the  other  hand,  electro-magnetic  waves  many  times  longer 
than  those  of  light,  and  producible  by  sudden  discharge  of  a  con- 
denser, could  not  be  utilized,  it  would  seem,  except  we  avail  our- 
selves of  their  effect  upon  conductors  as  in  the  present  methods, 
which  are  wasteful.  We  could  not  affect  by  means  of  such  waves 
the  static  molecular  or  atomic  charges  of  a  gas,  cause  them  to  vi- 
brate and  to  emit  light.  Long  transverse  waves  cannot,  apparently, 
produce  such  effects,  since  excessively  small  electro-magnetic 
disturbances  may  pass  readily  through  miles  of  air.  Such  dark 
waves,  unless  they  are  of  the  length  of  true  light  waves,  cannot, 
it  would  seem,  excite  luminous  radiation  in  a  Geissler  tube,  and 
the  luminous  effects,  which  are  producible  by  induction  in  a  tube 
devoid  of  electrodes,  I  am  inclined  to  consider  as  being  of  an  elec- 
trostatic nature. 

To  produce  such  luminous  effects,  straight  electrostatic  thrusts 
are  required;  these,  whatever  be  their  frequency,  may  disturb 
the  molecular  charges  and  produce  light.  Since  current  impulses 
of  the  required  frequency  cannot  pass  through  a  conductor  of 
measurable  dimensions,  we  must  work  with  a  gas,  and  then  the 
production  of  powerful  electrostatic  effects  becomes  an  imperative 
necessity. 

It  has  occurred  to  me,  however,  that  electrostatic  effects  are  in 
many  ways  available  for  the  production  of  light.  For  instance, 
we  may  place  a  body  of  some  refractory  material  in  a  closed,  and 
preferably  more  or  less  exhausted,  globe,  connect  it  to  a  source  of  • 
high,  rapidly  alternating  potential,  causing  the  molecules  of  the 
gas  to  strike  it  many  times  a  second  at  enormous  speeds,  and  in 
this  manner,  with  trillions  of  invisible  hammers,  pound  it  until  it 
gets  incandescent ;  or  we  may  place  a  body  in  a  very  highly  ex- 
hausted globe,  in  a  non-striking  vacuum,  and,  by  employing  very 


176  INVENTIONS  OF  NIKOLA  TE8LA. 

high  frequencies  and  potentials,  transfer  sufficient  energy  from  it 
to  other  bodies  in  the  vicinity,  or  in  general  to  the  surroundings, 
to  maintain  it  at  any  degree  of  incandescence ;  or  we  may,  by 
means  of  such  rapidly  alternating  high  potentials,  disturb  the 
ether  carried  by  the  molecules  of  a  gas  or  their  static  charges, 
causing  them  to  viorate  and  to  emit  light. 

But,  electrostatic  eifects  being  dependent  upon  the  potential 
and  frequency,  to  produce  the  most  powerful  action  it  is  desira- 
ble to  increase  both  as  far  as  practicable.  It  may  be  possible  to 
obtain  quite  fair  results  by  keeping  either  of  these  factors  small, 
provided  the  other  is  sufficiently  great ;  but  we  are  limited  in 
both  directions.  My  experience  demonstrates  that  we  cannot  go 
below  a  certain  frequency,  for,  first,  the  potential  then  becomes 
so  great  that  it  is  dangerous ;  and,  secondly,  the  light  production 
is  less  efficient. 

I  have  found  that,  by  using  the  ordinary  low  frequencies,  the 
physiological  effect  of  the  current  required  to  maintain  at  a  cer- 
tain degree  of  brightness  a  tube  four  feet  long,  provided  at  the 
ends  with  outside  and  inside  condenser  coatings,  is  so  powerful 
that,  I  think,  it  might  produce  serious  injury  to  those  not  accus- 
tomed to  such  shocks ;  whereas,  with  twenty  thousand  alterna- 
tions per  second,  the  tube  may  be  maintained  at  the  same  degree 
of  brightness  without  any  effect  being  felt.  This  is  due  princi- 
pally to  the  fact  that  a  much  smaller  potential  is  required  to  pro- 
duce the  same  light  effect,  and  also  to  the  higher  efficiency  in  the 
light  production.  It  is  evident  that  the  efficiencv  in  such  cases 
is  the  greater,  the  higher  the  frequency,  for  the  quicker  the  pro- 
cess of  charging  and  discharging  the  molecules,  the  less  energy 
will  be  lost  in  the  form  of  dark  radiation.  But,  unfortunately, 
we  cannot  go  beyond  a  certain  frequency  on  account  of  the  diffi- 
culty of  producing  and  conveying  the  effects. 

I  have  stated  above  that  a  body  inclosed  in  an  unexhausted 
bulb  may  be  intensely  heated  by  simply  connecting  it  with  a 
source  of  rapidly  alternating  potential.  The  heating  in  such  a 
case  is,  in  all  probability,  due  mostly  to  the  bombardment  of  the 
molecules  of  the  gas  contained  in  the  bulb.  When  the  bulb  is 
exhausted,  the  heating  of  the  body  is  much  more  rapid,  and  there 
is  no  difficulty  whatever  in  bringing  a  wire  or  filament  to  any 
degree  of  incandescence  by  simply  connecting  it  to  one  terminal 
of  a  coil  of  the  proper  dimensions.  Thus,  if  the  well-known  ap- 
paratus of  Prof.  Crookes,  consisting  of  a  bent  platinum  wire  with 


HIGH  FREQUENCY  AND  HIGH  POTENTIAL  CURRENTS.      177 

vanes  mounted  over  it  (Fig.  114),  be  connected  to  one  terminal  of 
the  coil — either  one  or  both  ends  of  the  platinum  wire  being  con- 
nected— the  wire  is  rendered  almost  instantly  incandescent,  and 
the  mica  vanes  are  rotated  as  though  a  current  from  a  battery 
were  used.  A  thin  carbon  filament,  or,  preferably,  a  button  of 
some  refractory  material  (Fig.  115),  even  if  it  be  a  comparatively 
poor  conductor,  inclosed  in  an  exhausted  globe,  may  be  rendered 
highly  incandescent ;  and  in  this  manner  a  simple  lamp  capable 
of  giving  any  desired  candle  power  is  provided. 

The  success  of  lamps  of  this  kind  would  depend  largely  on  the 
selection  of  the  light-giving  bodies  contained  within  the  bulb. 
Since,  under  the  conditions  described,  refractory  bodies — which 
are  very  poor  conductors  and  capable  of  withstanding  for  a  long 
time  excessively  high  degrees  of  temperature — may  be  used, 
such  illuminating  devices  may  be  rendered  successful. 

It  might  be  thought  at  first  that  if  the  bulb,  containing  the 


FIG.  114.  FIG.  115. 

filament  or  button  of  refractory  material,  be  perfectly  well  ex- 
hausted— that  is,  as  far  as  it  can  be  done  by  the  use  of  the  best 
apparatus — the  heating  would  be  much  less  intense,  and  that  in 
a  perfect  vacuum  it  could  not  occur  at  all.  This  is  not  confirmed 
by  my  experience;  quite  the  contrary,  the  better  the  vacuum 
the  more  easily  the  bodies  are  brought  to  incandescence.  This 
result  is  interesting  for  many  reasons. 

At  the  outset  of  this  work  the  idea  presented  itself  to  me, 
whether  two  bodies  of  refractory  material  enclosed  in  a  bulb  ex- 
hausted to  such  a  degree  that  the  discharge  of  a  large  induction 
coil,  operated  in  the  usual  manner,  cannot  pass  through,  could  be 
rendered  incandescent  by  mere  condenser  action.  Obviously,  to 
reach  this  result  enormous  potential  differences  and  very  high 
frequencies  are  required,  as  is  evident  from  a  simple  calcula- 
tion. 


178  INVENTIONS  OF  NIKOLA  TESLA. 

But  such  a  lamp  would  possess  a  vast  advantage  over  an  ordi- 
nary incandescent  lamp  in  regard  to  efficiency.  It  is  well-known 
that  the  efficiency  of  a  lamp  is  to  some  extent  a  function  of  the 
degree  of  incandescence,  and  that,  could  we  but  work  a  filament 
at  many  times  higher  degrees  of  incandescence,  the  efficiency 
would  be  much  greater.  In  an  ordinary  lamp  this  is  impractic- 
able on  account  of  the  destruction  of  the  filament,  and  it  has  been 
determined  by  experience  how  far  it  is  advisable  to  push  the  in- 
candescence. It  is  impossible  to  tell  how  much  higher  efficiency 
could  be  obtained  if  the  filament  could  withstand  indefinitely, 
as  the  investigation  to  this  end  obviously  cannot  be  carried  be- 
yond a  certain  stage  ;  but  there  are  reasons  for  believing  that  it 
would  be  very  considerably  higher.  An  improvement  might  be 
made  in  the  ordinary  lamp  by  employing  a  short  and  thick  car- 
bon ;  but  then  the  leading-in  wires  would  have  to  be  thick,  and, 
besides,  there  are  many  other  considerations  which  render  such  a 
modification  entirely  impracticable.  But  in  a  lamp  as  above  de- 
scribed, the  leading  in  wires  may  be  very  small,  the  incandescent 
refractory  material  may  be  in  the  shape  of  blocks  offering  a  very 
small  radiating  surface,  so  that  less  energy  would  be  required  to 
keep  them  at  the  desired  incandescence  ;  and  in  addition  to  this, 
the  refractory  material  need  not  be  carbon,  but  may  be  manufac- 
tured from  mixtures  of  oxides,  for  instance,  with  carbon  or  other 
material,  or  may  be  selected  from  bodies  which  are  practically 
non-conductors,  and  capable  of  withstanding  enormous  degrees  of 
temperature. 

All  this  would  point  to  the  possibility  of  obtaining  a  much 
higher  efficiency  with  such  a  lamp  than  is  obtainable  in  ordinary 
lamps.  In  my  experience  it  has  been  demonstrated  that  the 
blocks  are  brought  to  high  degrees  of  incandescence  with  much 
lower  potentials  than  those  determined  by  calculation,  and  the 
blocks  may  be  set  at  greater  distances  from  each  other.  We  may 
freely  assume,  and  it  is  probable,  that  the  molecular  bombard- 
ment is  an  important  element  in  the  heating,  even  if  the  globe 
be  exhausted  with  the  utmost  care,  as  I  have  done ;  for  although 
the  number  of  the  molecules  is,  comparatively  speaking,  insign- 
ificant, yet  on  account  of  the  mean  free  path  being  very  great, 
there  are  fewer  collisions,  and  the  molecules  may  reach  much 
higher  speeds,  so  that  the  heating  effect  due  to  this  cause  may 
be  considerable,  as  in  the  Crookes  experiments  with  radiant 
matter. 


HIGH  FREQUENCY  AND  HIGH  POTENTIAL  VUlillENTt*.      179 


But  it  is  likewise  possible  that  we  have  to  deal  here  with  an 
increased  facility  of  losing  the  charge  in  very  high  vacuum,  when 
the  potential  is  rapidly  alternating,  in  which  case  most  of  the 
heating  would  be  directly  due  to  the  surging  of  the  charges  in 
the  heated  bodies.  Or  else  the  observed  fact  may  be  largely 
attributable  to  the  effect  of  the  points  which  I  have  mentioned 
above,  in  consequence  of  which  the  blocks  or  filaments  contained 
in  the  vacuum  are  equivalent  to  condensers  of  many  times 
greater  surface  than  that  calculated  from  their  geometrical  dimen- 
sion^. Scientific  men  still  differ  in  opinion  as  to  whether  a 
charge  should,  or  should  not,  be  lost  in  a  perfect  vacuum,  or  in 
other  words,  whether  ether  is,  or  is  not,  a  conductor.  If  the 


FIG.  116. 


FIG.  117. 


former  were  the  case,  then  a  thin  filament  enclosed  in  a  perfectly 
exhausted  globe,  and  connected  to  a  source  of  enormous,  steady 
potential,  would  be  brought  to  incandescence. 

Various  forms  of  lamps  on  the  above  described  principle,  with 
the  refractory  bodies  in  the  form  of  filaments,  Fig.  116,  or  blocks, 
Fig.  117,  have  been  constructed  and  operated  by  me,  and  investi- 
gations are  being  carried  on  in  this  line.  There  is  no  difficulty  in 
reaching  such  high  degrees  of  incandescence  that  ordinary  car- 
bon is  to  all  appearance  melted  and  volatilized.  If  the  vacuum 
could  be  made  absolutely  perfect,  such  a  lamp,  although  inopera- 
tive with  apparatus  ordinarily  used,  would,  if  operated  with  cur- 


180  INVENTIONS  OF  NIKOLA  TESLA. 

rents  of  the  required  character,  afford  an  illuminant  which  would 
never  be  destroyed,  and  which  would  be  far  more  efficient  than 
an  ordinary  incandescent  lamp.  This  perfection  can,  of  course, 
never  be  reached,  and  a  very  slow  destruction  and  gradual  diminu- 
tion in  size  always  occurs,  as  in  incandescent  filaments  ;  but  there 
is  no  possibility  of  a  sudden  and  premature  disabling  which  oc- 
curs in  the  latter  by  the  breaking  of  the  filament,  especially 
when  the  incandescent  bodies  are  in  the  shape  of  blocks. 

With  these  rapidly  alternating  potentials  there  is,  however,  no 
necessity  of  enclosing  two  blocks  in  a  globe,  but  a  single  block, 
as  in  Fig.  115,  or  filament,  Fig.  118,  may  be  used.  The  poten- 
tial in  this  case  must  of  course  be  higher,  but  is  easily  obtainable, 
and  besides  it  is  not  necessarily  dangerous. 

The  facility  with  which  the  button  or  filament  in  such  a  lamp 


FIG.  118. 

is  brought  to  incandescence,  other  things  being  equal,  depends 
on  the  size  of  the  globe.  If  a  perfect  vacuum  could  be  obtained, 
the  size  of  the  globe  would  not  be  of  importance,  for  then  the 
heating  would  be  wholly  due  to  the  surging  of  the  charges,  and 
all  the  energy  would  be  given  off  to  the  surroundings  by  radia- 
tion. But  this  can  never  occur  in  practice.  There  is  always 
some  gas  left  in  the  globe,  and  although  the  exhaustion  may  be 
carried  to  the  highest  degree,  still  the  space  inside  of  the  bulb 
must  be  considered  as  conducting  when  such  high  potentials  are 
used,  and  I  assume  that,  in  estimating  the  energy  that  may  be 
given  off  from  the  filament  to  the  surroundings,  we  may  consider 


HIGH  FREQUENCY  AND  HIGH  POTENTIAL  CURREN1S.      181 

the  inside  surface  of  the  bulb  as  one  coating  of  a  condenser,  the 
air  and  other  objects  surrounding  the  bulb  forming  the  other 
coating.  When  the  alternations  are  very  low  there  is  no  doubt 
that  a  considerable  portion  of  the  energy  is  given  off  by  the  elec- 
trification, of  the  surrounding  air. 

In  order  to  study  this  subject  better,  I  carried  on  some  experi- 
ments with  excessively  high  potentials  and  low  frequencies.  I 
then  observed  that  when  the  hand  is  approached  to  the  bulb, — 
the  filament  being  connected  with  one  terminal  of  the  coil, — a 
powerful  vibration  is  felt,  being  due  to  the  attraction  and  repul- 
sion of  the  molecules  of  the  air  which  are  electrified  by  induc- 
tion through  the  glass.  In  some  cases  when  the  action  is  very 
intense  I  have  been  able  to  hear  a  sound,  which  must  be  due  to 
the  same  cause. 

When  the  alternations  are  low,  one  is  agt  to  get  an  excessively 


FIG.  119.  FIG.  120. 

powerful  shock  from  the  bulb.  In  general,  when  one  attaches 
bulbs  or  objects  of  some  size  to  the  terminals  of  the  coil,  one 
should  look  out  for  the  rise  of  potential,  for  it  may  happen  that 
by  merely  connecting  a  bulb  or  plate  to  the  terminal,  the  poten- 
tial may  rise  to  many  times  its  original  value.  When  lamps  are 
attached  to  the  terminals,  as  illustrated  in  Fig.  119,  then  the 
capacity  of  the  bulbs  should  be  such  as  to  give  the  maximum 
rise  of  potential  under  the  existing  conditions.  In  this  man- 
ner one  may  obtain  the  required  potential  with  fewer  turns  of 
wire. 

The  life  of  such  lamps  as  described  above  depends,  of  course, 
largely  on  the  degree  of  exhaustion,  but  to  some  extent  also  on 
the  shape  of  the  block  of  refractory  material.  Theoretically  it 


INVENTIONS  OF  NIKOLA  TESLA. 


would  seem  that  a  small  sphere  of  carbon  enclosed  in  a  sphere  of 
glass  would  not  suffer  deterioration  from  molecular  bombard- 
ment, for,  the  matter  in  the  globe  being  radiant,  the  molecules 
would  move  in  straight  lines,  and  would  seldom  strike  the  sphere 
obliquely.  An  interesting  thought  in  connection  with  such  a 
lamp  is,  that  in  it  "  electricity "  and  electrical  energy  apparently 
must  move  in  the  same  lines. 

The  use  of  alternating  currents  of  very  high  frequency  makes 
it  possible  to  transfer,  by  electrostatic  or  electromagnetic  induc- 
tion through  the  glass  of  a  lamp,  sufficient  energy  to  keep  a  fila- 


FIG.  121a. 


FIG.  121b. 


ment  at  incandescence  and  so  do  away  with  the  leading-in  wires. 
Such  lamps  have  been  proposed,  but  for  want  of  proper  appara- 
tus they  have  not  been  successfully  operated.  Many  forms  of 
lamps  on  this  principle  with  continuous  and  broken  filaments 
have  been  constructed  by  me  and  experimented  upon.  When 
using  a  secondary  enclosed  within  the  lamp,  a  condenser  is  ad- 
vantageously combined  with  the  secondary.  When  the  transfer- 
ence is  effected  by  electrostatic  induction,  the  potentials  used  are, 
of  course,  very  high  with  frequencies  obtainable  from  a  machine. 
For  instance,  with  a  condenser  surface  of  forty  square  centimetres, 


HIGH  FREQUENCY  AND  HIGH  POTENTIAL  CURRENTS.      183 

which  is  not  impracticably  large,  and  with  glass  of  good  quality 
1  ram.  thick,  using  currents  alternating  twenty  thousand  times 
a  second,  the  potential  required  is  approximately  9,000  volts. 
This  may  seem  large,  but  since  each  lamp  may  be  included 
in-  the  secondary  of  a  transformer  of  very  small  dimensions,  it 
would  not  be  inconvenient,  and,  moreover,  it  would  not  produce 
fatal  injury.  The  transformers  would  all  be  preferably  in  series. 
The  regulation  would  offer  no  difficulties,  as  with  currents  of  such 
frequencies  it  is  very  easy  to  maintain  a  constant  current. 

In  the  accompanying  engravings  some  of  the  types  of  lamps  of 
this  kind  are  shown.  Fig.  120  is  such  a  lamp  with  a  broken  fila- 
ment, and  Figs.  121  A  and  121  B  one  with  a  single  outside  and 
inside  coating  and  a  single  filament.  I  have  also  made  lamps 
with  two  outside  and  inside  coatings  and  a  continuous  loop  con- 
necting the  latter.  Such  lamps  have  been  operated  by  me  with 
current  impulses  of  the  enormous  frequencies  obtainable  by  the 
disruptive  discharge  of  condensers. 

The  disruptive  discharge  of  a  condenser  is  especially  suited  for 
operating  such  lamps — with  no  outward  electrical  connections — 
by  means  of  electromagnetic  induction,  the  electromagnetic  in- 
ductive effects  being  excessively  high ;  and  I  have  been  able  to 
produce  the  desired  incandescence  with  only  a  few  short  turns  of 
wire.  Incandescence  may  also  be  produced  in  this  manner  in  a 
simple  closed  filament. 

Leaving  now  out  of  consideration  the  practicability  of  such 
lamps,  I  would  only  say  that  they  possess  a  beautiful  and  desir- 
able feature,  namely,  that  they  can  be  rendered,  at  will,  more  or 
less  brilliant  simply  by  altering  the  relative  position  of  the  out- 
side and  inside  condenser  coatings,  or  inducing  and  induced  cir- 
cuits. 

When  a  lamp  is  lighted  by  connecting  it  to  one  terminal  only 
of  the  source,  this  may  be  facilitated  by  providing  the  globe  with 
an  outside  condenser  coating,  which  serves  at  the  same  time  as  a 
reflector,  and  connecting  this  to  an  insulated  body  of  some  size. 
Lamps  of  this  kind  are  illustrated  in  Fig.  122  and  Fig.  123. 
Fig.  124  shows  the  plan  of  connection.  The  brilliancy  of  the 
lamp  may,  in  this  case,  be  regulated  within  wide  limits  by  vary- 
ing the  size  of  the  insulated  metal  plate  to  which  the  coating  is 
connected. 

It  is  likewise  practicable  to  light  with  one  leading  wire  lamps 
such  as  illustrated  in  Fig.  116  and  Fig.  117,  by  connecting  one 


184  INVENTIONS  OF  NIKOLA  TKSLA. 

terminal  of  the  lamp  to  one  terminal  of  the  source,  and  the 
other  to  an  insulated  body  of  the  required  size.  In  all  cases 
the  insulated  body  serves  to  give  off  the  energy  into  the  sur- 
rounding space,  and  is  equivalent  to  a  return  wire.  Obviously, 
in  the  two  last-named  cases,  instead  of  connecting  the  wires  to 
an  insulated  body,  connections  may  be  made  to  the  ground. 

The  experiments  which  will  prove  most  suggestive  and  of 
most  interest  to  the  investigator  are  probably  those  performed 
with  exhausted  tubes.  As  might  be  anticipated,  a  source  of  such 
rapidly  alternating  potentials  is  capable  of  exciting  the  tubes  at 
a  considerable  distance,  and  the  light  effects  produced  are  re- 
markable. 

During  my  investigations  in  this  line  I  endeavored  to  excite 


FIG.  122.  FIG.  123. 

tubes,  devoid  of  any  electrodes,  by  electromagnetic  induction, 
making  the  tube  the  secondary  of  the  induction  device,  and 
passing  through  the  primary  the  discharges  of  a  Leyden  jar. 
These  tubes  were  made  of  many  shapes,  and  I  was  able  to 
obtain  luminous  effects  which  I  then  thought  were  due  wholly 
to  electromagnetic  induction.  But  on  carefully  investigating 
the  phenomena  I  found  that  the  effects  produced  were  more 
of  an  electrostatic  nature.  It  may  be  attributed  to  this  cir- 
cumstance that  this  mode  of  exciting  tubes  is  very  wasteful, 
namely,  the  primary  circuit  being  closed,  the  potential,  and 
consequently  the  electrostatic  inductive  effect,  is  much  dimin- 
ished. 


HIGH  FREQUENCY  AND  HIGH  POTENTIAL  CURRENTS.      185 

When  an  induction  coil,  operated  as  above  described,  is  used, 
there  is  no  doubt  that  the  tubes  are  excited  by  electrostatic  in- 
duction, and  that  electromagnetic  induction  has  little,  if  any- 
thing, to  do  with  the  phenomena. 

This  is  evident  from  many  experiments.  For  instance,  if  a 
tube  be  taken  in  one  hand,  the  observer  being  near  the  coil,  it  is 
brilliantly  lighted  and  remains  so  no  matter  in  what  position  it  is 
held  relatively  to  the  observer's  body.  Were  the  action  electro- 
magnetic, the  tube  could  not  be  lighted  when  the  observer's 
body  is  interposed  between  it  and  the  coil,  or  at  least  its  lumi- 
nosity should  be  considerably  diminished.  When  the  tube  is 
held  exactly  over  the  centre  of  the  coil — the  latter  being  wound 
in  sections  and  the  primary  placed  symmetrically  to  the  sec- 
ondary— it  may  remain  completely  dark,  whereas  it  is  rendered 
intensely  luminous  by  moving  it  slightly  to  the  right  or  left 
from  the  centre  of  the  coil.  It  does  not  light  because  in  the 


FIG.  124. 

middle  both  halves  of  the  coil  neutralize  each  other,  and  the 
electric  potential  is  zero.  If  the  action  were  electromagnetic, 
the  tube  should  light  best  in  the  plane  through  the  centre  of  the 
coil,  since  the  electromagnetic  effect  there  should  be  a  maximum. 
When  an  arc  is  established  between  the  terminals,  the  tubes  and 
lamps  in  the  vicinity  of  the  coil  go  out,  but  light  up  again 
when  the  arc  is  broken,  on  account  of  the  rise  of  potential.  Yet 
the  electromagnetic  effect  should  be  practically  the  same  in  both 
cases. 

By  placing  a  tube  at  some  distance  from  the  coil,  and  nearer  to 
one  terminal — preferably  at  a  point  on  the  axis  of  the  coil — one 
may  light  it  by  touching  the  remote  terminal  with  an  insulated 
body  of  some  size  or  with  the  hand,  thereby  raising  the  potential 
at  that  terminal  nearer  to  the  tube.  If  the  tube  is  shifted  nearer 
to  the  coil  so  that  it  is  lighted  by  the  action  of  the  nearer  termi- 


186  INVENTIONS  OF  NIKOLA  TEBLA. 

nal,  it  may  be  made  to  go  out  by  holding,  on  an  insulated  sup- 
port, the  end  of  a  wire  connected  to  the  remote  terminal,  in  the 
vicinity  of  the  nearer  terminal,  by  this  means  counteracting  the 
action  of  the  latter  upon  the  tube.  These  effects  are  evidently 
electrostatic.  Likewise,  when  a  tube  is  placed  at  a  considerable 
distance  from  the  coil,  the  observer  may,  standing  upon  an  insu- 
lated support  between  coil  and  tube,  light  the  latter  by  approach- 
ing the  hand  to  it ;  or  he  may  even  render  it  luminous  by  simply 
stepping  between  it  and  the  coil.  This  would  be  impossible  with 
electro-magnetic  induction,  for  the  body  of  the  observer  would 
act  as  a  screen. 

When  the  coil  is  energized  by  excessively  weak  currents,  the 
experimenter  may,  by  touching  one  terminal  of  the  coil  with  the 
tube,  extinguish  the  latter,  and  may  again  light  it  by  bringing  it 
out  of  contact  with  the  terminal  and  allowing  a  small  arc  to  form. 
This  is  clearly  due  to  the  respective  lowering  and  raising  of  the 
potential  at  that  terminal.  In  the  above  experiment,  when  the 
tube  is  lighted  through  a  small  arc,  it  may  go  out  when  the  arc  is 
broken,  because  the  electrostatic  inductive  effect  alone  is  too 
weak,  though  the  potential  may  be  much  higher ;  but  when  the 
arc  is  established,  the  electrification  of  the  end  of  the  tube  is 
much  greater,  and  it  consequently  lights. 

If  a  tube  is  lighted  by  holding  it  near  to  the  coil,  and  in  the 
hand  which  is  remote,  by  grasping  the  tube  anywhere  with  the 
other  hand,  the  part  between  the  hands  is  rendered  dark,  and  the 
singular  effect  of  wiping  out  the  light  of  the  tube  may  be  pro- 
duced by  passing  the  hand  quickly  along  the  tube  and  at  the 
same  time  withdrawing  it  gently  from  the  coil,  judging  prop- 
erly the  distance  so  that  the  tube  remains  dark  afterwards. 

If  the  primary  coil  is  placed  sidewise,  as  in  Fig.  112  B  for  in- 
stance, and  an  exhausted  tube  be  introduced  from  the  other  side 
in  the  hollow  space,  the  tube  is  lighted  most  intensely  because  of 
the  increased  condenser  action,  and  in  this  position  the  striae  are 
most  sharply  defined.  In  all  these  experiments  described,  and  in 
many  others,  the  action  is  clearly  electrostatic. 

The  effects  of  screening  also  indicate  the  electrostatic  nature 
of  the  phenomena  and  show  something  of  the  nature  of  electri- 
fication through  the  air.  For  instance,  if  a  tube  is  placed  in  the 
direction  of  the  axis  of  the  coil,  and  an  insulated  metal  plate  be 
interposed,  the  tube  will  generally  increase  in  brilliancy,  or  if  it 
l>e  too  far  from  the  coil  to  light,  it  may  even  be  rendered  lumin- 


HIGH  FREQUENCY  AND  HIGH  POTENTIAL  CURRENTS.      187 

ous  by  interposing  an  insulated  metal  plate.  The  magnitude  of 
the  effects  depends  to  some  extent  on  the  size  of  the  plate.  But  if 
the  metal  plate  be  connected'by  a  wire  to  the  ground,  its  interpo- 
sition will  always  make  the  tube  go  out  even  if  it  be  very  near  the 
coil.  In  general,  the  interposition  of  a  body  between  the  coil  and 
tube,  increases  or  diminishes  the  brilliancy  of  the  tube,  or  its 
facility  to  light  up,  according  to  whether  it  increases  or  dimin- 
ishes the  electrification.  When  experimenting  with  an  insulated 
plate,  the  plate  should  not  be  taken  too  large,  else  it  will  generally 
produce  a  weakening  effect  by  reason  of  its  great  facility  for  giv- 
ing off  energy  to  the  surroundings. 

If  a  tube  be  lighted  at  some  distance  from  the  coil,  and  a  plate 
of  hard  rubber  or  other  insulating  substance  be  interposed,  the 
tube  may  be  made  to  go  out.  The  interposition  of  the  dielectric 
in  this  case  only  slightly  increases  the  inductive  effect,  but  dimin- 
ishes considerably  the  electrification  through  the  air. 

In  all  cases,  then,  when  we  excite  luminosity  in  exhausted 
tubes  by  means  of  such  a  coil,  the  effect  is  due  to  the  rapidly 
alternating  electrostatic  potential ;  and,  furthermore,  it  must  be 
attributed  to  the  harmonic  alternation  produced  directly  by  the 
machine,  and  not  to  any  superimposed  vibration  which  might  be 
thought  to  exist.  Such  superimposed  vibrations  are  impossible 
when  we  work  with  an  alternate  current  machine.  If  a  spring  be 
gradually  tightened  and  released,  it  does  not  perform  independ- 
ent vibrations ;  for  this  a  sudden  release  is  necessary.  So  with 
the  alternate  currents  from  a  dynamo  machine ;  the  medium  is 
harmonically  strained  and  released,  this  giving  rise  to  only  one 
kind  of  waves ;  a  sudden  contact  or  break,  or  a  sudden  giving 
way  of  the  dielectric,  as  in  the  disruptive  discharge  of  a  Leyden 
jar,  are  essential  for  the  production  of  superimposed  waves. 

In  all  the  last  described  experiments,  tubes  devoid  of  any  elec- 
trodes may  be  used,  and  there  is  no  difficulty  in  producing  by 
their  means  sufficient  light  to  read  by.  The  light  effect  is,  how- 
ever, considerably  increased  by  the  use  of  phosphorescent  bodies 
such  as  yttria,  uranium  glass,  etc.  A  difficulty  will  be  found 
when  the  phosphorescent  material  is  used,  for  with  these  power- 
ful effects,  it  is  carried  gradually  away,  and  it  is  preferable  to  use 
material  in  the  form  of  a  solid. 

Instead  of  depending  on  induction  at  a  distance  to  light  the 
tube,  the  same  may  be  provided  with  an  external — and,  if  de- 
sired, also  with  an  internal — condenser  coating,  and  it  may  then 


188 


INVENTIONS  OF  NIKOLA  TESLA. 


be  suspended  anywhere  in  the  room  from  a  conductor  connected 
to  one  terminal  of  the  coil,  and  in  this  manner  a  soft  illumination 
may  be  provided. 

The  ideal  way  of  lighting  a  hall  or  room  would,  however,  be 


FIG.  125. 


to  produce  such  a  condition  in  it  that  an  illuminating  device 
could  be  moved  and  put  anywhere,  and  that  it  is  lighted,  no  mat- 
ter where  it  is  put  and  without  being  electrically  connected  to 


HIGH  FREQUENCY  AND  HIGH  POTENTIAL  CURRENTS.      189 

anything.  I  have  been  able  to  produce  such  a  condition  by  creat- 
ing in  the  room  a  powerful,  rapidly  alternating  electrostatic 
field.  For  this  purpose  I  suspend  a  sheet  of  metal  a  distance 
from  the  ceiling  on  insulating  cords  and  connect  it  to  one  termi- 
nal of  the  induction  coil,  the  other  terminal  being  preferably  con- 
nected to  the  ground.  Or  else  I  suspend  two  sheets  as  illustrated 
in  Fig.  125,  each  sheet  being  connected  with  one  of  the  terminals 
of  the  coil,  and  their  size  being  carefully  determined.  An  ex- 
hausted tube  may  then  be  carried  in  the  hand  anywhere  be- 
tween the  sheets  or  placed  anywhere,  even  a  certain  distance 
beyond  them  ;  it  remains  always  luminous. 

In  such  an  electrostatic  field  interesting  phenomena  may  be 
observed,  especially  if  the  alternations  are  kept  low  and  the  po- 
tentials excessively  high.  In  addition  to  the  luminous  phenomena 
mentioned,  one  may  observe  that  any  insulated  conductor  gives 
sparks  when  the  hand  or  another  object  is  approached  to  it,  and 
the  sparks  may  often  be  powerful.  When  a  large  conducting 
object  is  fastened  on  an  insulating  support,  and  the  hand  ap- 
proached to  it,  a  vibration,  due  to  the  rythmical  motion  of  the 
air  molecules  is  felt,  and  luminous  streams  may  be  perceived 
when  the  hand  is  held  near  a  pointed  projection.  '  When  a  tele- 
phone receiver  is  made  to  touch  with  one  or  both  of  its  terminals 
an  insulated  conductor  of  some  size,  the  telephone  emits  a  loud 
sound ;  it  also  emits  a  sound  when  a  length  of  wire  is  attached  to 
one  or  both  terminals,  and  with  very  powerful  fields  a  sound  may 
be  perceived  even  without  any  wire. 

How  far  this  principle  is  capable  of  practical  application,  the 
future  will  tell.  It  might  be  thought  that  electrostatic  effects 
are  unsuited  for  such  action  at  a  distance.  Electromagnetic  in- 
ductive effects,  if  available  for  the  production  of  light,  might  be 
thought  better  suited.  It  is  true  the  electrostatic  effects  dimin- 
ish nearly  with  the  cube  of  the  distance  from  the  coil,  whereas 
the  electromagnetic  inductive  effects  diminish  simply  with  the 
distance.  But  when  we  establish  an  electrostatic  field  of  force, 
the  condition  is  very  different,  for  then,  instead  of  the  differen- 
tial effect  of  both  the  terminals,  we  get  their  conjoint  effect. 
Besides,  I  would  call  attention  to  the  effect,  that  in  an  alternat- 
ing electrostatic  field,  a  conductor,  such  as  an  exhausted  tube? 
for  instance,  tends  to  take  up  most  of  the  energy,  whereas  in  an 
electromagnetic  alternating  field  the  conductor  tends  to  take  up 
tlie  least  energy,  the  waves  being  reflected  with  but  little- loss. 


190  INVENTIONS  OF  NIKOLA  TESLA. 

This  is  one  reason  why  it  is  difficult  to  excite  an  exhausted  tube, 
at  a  distance,  by  electromagnetic  induction.  I  have  wound  coils 
of  very  large  diameter  and  of  many  turns  of  wire,  and  connected 
a  Geissler  tube  to  the  ends  of  the  coil  with  the  object  of  exciting 
the  tube  at  a  distance ;  but  even  with  the  powerful  inductive 
effects  producible  by  Ley  den  jar  discharges,  the  tube  could  not 
be  excited  unless  at  a  very  small  distance,  although  some  judg- 
ment was  used  as  to  the  dimensions  of  the  coil.  I  have  also 
found  that  even  the  most  powerful  Leyden  jar  discharges  are 
capable  of  exciting  only  feeble  luminous  effects  in  a  closed  ex- 
hausted tube,  and  even  these  effects  upon  thorough  examination 
I  have  been  forced  to  consider  of  an  electrostatic  nature. 

How  then  can  we  hope  to  produce  the  required  effects  at  a 
distance  by  means  of  electromagnetic  action,  when  even  in  the 
closest  proximity  to  the  source  of  disturbance,  under  the  most 
advantageous  conditions,  we  can  excite  but  faint  luminosity  '*  It 
is  true  that  when  acting  at  a  distance  we  have  the  resonance  to 
help  us  out.  We  can  connect  an  exhausted  tube,  or  whatever 
the  illuminating  device  may  be,  with  an  insulated  system  of  the 
proper  capacity,  and  so  it  may  be  possible  to  increase  the  effect 
qualitatively,  and  only  qualitatively,  for  we  would  not  get  more 
energy  through  the  device.  So  we  may,  by  resonance  effect, 
obtain  the  required  electromotive  force  in  an  exhausted  tube,  and 
excite  faint  luminous  effects,  but  we  cannot  get  enough  energy  to 
render  the  light  practically  available,  and  a  simple  calculation, 
based  on  experimental  results,  shows  that  even  if  all  the  energy 
which  a  tube  would  receive  at  a  certain  distance  from  the  source 
should  be  wholly  converted  into  light,  it  would  hardly  satisfy  the 
practical  requirements.  Hence  the  necessity  of  directing,  by 
means  of  a  conducting  circuit,  the  energy  to  the  place  of  trans- 
formation. But  in  so  doing  we  cannot  very  sensibly  depart  from 
present  methods,  and  all  we  could  do  would  be  to  improve  the1 
apparatus. 

From  these  considerations  it  would  seem  that  if  this  ideal  way 
of  lighting  is  to  be  rendered  practicable  it  will  be  only  by  the  use 
of  electrostatic  effects.  In  such  a  case  the  most  powerful  electro- 
static inductive  effects  are  needed ;  the  apparatus  employed  must, 
therefore,  be  capable  of  producing  high  electrostatic  potentials 
changing  in  value  with  extreme  rapidity.  High  frequencies  are 
especially  wanted,  for  practical  considerations  make  it  desirable 
to  keep  down  the  potential.  By  the  employment  of  machines, 


HIGH  FREQUENCY  AND  HIGH  POTENTIAL  CURRENTS.      191 

or,  generally  speaking,  of  any  mechanical  apparatus,  but  low 
frequencies  can  be  reached  ;  recourse  must,  therefore,  be  had  to 
some  other  means.  The  discharge  of  a  condenser  affords  us  a 
means  of  obtaining  frequencies  by  far  higher  than  are  obtainable 
mechanically,  and  I  have  accordingly  employed  condensers  in  the 
experiments  to  the  above  end. 

When  the  terminals  of  a  high  tension  induction  coil,  Fig.  120, 
are  connected  to  a  Leyden  jar,  and  the  latter  is  discharging  dis- 
ruptively  into  a  circuit,  we  may  look  upon  the  arc  playing  be- 
tween the  knobs  as  being  a  source  of  alternating,  or  generally 
speaking,  undulating  currents,  and  then  we  have  to  deal  with 
the  familiar  system  of  a  generator  of  such  currents,  a  circuit  con- 
nected to  it,  and  a  condenser  bridging  the  circuit.  The  condenser 
in  such  case  is  a  veritable  transformer,  and  since  the  frequency  is 
excessive,  almost  any  ratio  in  the  strength  of  the  currents  in  both 
the  branches  may  be  obtained.  In  reality  the  analogy  is  not  quite 
complete,  for  in  the  disruptive  discharge  we  have  most  generally 
a  fundamental  instantaneous  variation  of  comparatively  low  fre- 
quency, and  a  superimposed  harmonic  vibration,  and  the  laws 
governing  the  flow  of  currents  are  not  the  same  for  both. 

In  converting  in  this  manner,  the  ratio  of  conversion  should 
not  be  too  great,  for  the  loss  in  the  arc  between  the  knobs  in- 
creases with  the  square  of  the  current,  and  if  the  jar  be  discharged 
through  very  thick  and  short  conductors,  with  the  view  of  ob- 
taining a  very  rapid  oscillation,  a  very  considerable  portion  of  the 
energy  stored  is  lost.  On  the  other  hand,  too  small  ratios  are  not 
practicable  for  many  obvious  reasons. 

As  the  converted  currents  flow  in  a  practically  closed  circuit, 
the  electrostatic  effects  are  necessarily  small,  and  I  therefore  con- 
vert them  into  currents  or  effects  of  the  required  character.  I 
have  effected  such  conversions  in  several  ways.  The  preferred 
plan  of  connections  is  illustrated  in  Fig.  127.  The  manner  of  oper- 
ating renders  it  easy  to  obtain  by  means  of  a  small  and  inexpen- 
sive apparatus  enormous  differences  of  potential  which  have  been 
usually  obtained  by  means  of  large  and  expensive  coils.  For  this 
it  is  only  necessary  to  take  an  ordinary  small  coil,  adjust  to  it  a 
condenser  and  discharging  circuit,  forming,  the  primary  of  an 
auxiliary  small  coil,  and  convert  upward.  As  the  inductive  effect 
of  the  primary  currents  is  excessively  great,  the  second  coil  need 
have  comparatively  but  very  few  turns.  By  properly  adjusting 
the  elements,  remarkable  results  may  be  secured. 


192  INVENTIONS  OF  NIKOLA  TESLA. 

In  endeavoring  to  obtain  tlie  required  electrostatic  effects  in 
this  manner,  I  have,  as  might  be  expected,  encountered  many 
difficulties  which  I  have  been  gradually  overcoming,  but  I  am  not 
as  yet  prepared  to  dwell  upon  my  experiences  in  this  direction. 

I  believe  that  the  disruptive  discharge  of  a  condenser  will  play 
an  important  part  in  the  future,  for  it  offers  vast  possibilities, 
not  only  in  the  way  of  producing  light  in  a  more  efficient  manner 
and  in  the  line  indicated  by  theory,  but  also  in  many  other  re- 
spects. 

For  years  the  efforts  of  inventors  have  been  directed  towards 
obtaining  electrical  energy  from  heat  by  means  of  the  thermo- 
pile. It  might  seem  invidious  to  remark  that  but  few  know 
what  is  the  real  trouble  with  the  thermopile.  It  is  not  the  in- 
efficiency or  small  output — though  these  are  great  drawbacks — 
but  the  fact  that  the  thermopile  has  its  phylloxera,  that  is,  that 
by  constant  use  it  is  deteriorated,  which  has  thus  far  prevented  its 


FIG.  126. 

introduction  on  an  industrial  scale.  Now  that  all  modern  re- 
search seems  to  point  with  certainty  to  the  use  of  electricity  of  ex- 
cessively high  tension,  the  question  must  present  itself  to  many 
whether  it  is  not  possible  to  obtain  in  a  practicable  manner  this 
form  of  energy  from  heat.  We  have  been  used  to  look  upon 
an  electrostatic  machine  as  a  plaything,  and  somehow  we  couple 
with  it  the  idea  of  the  inefficient  and  impractical.  But  now  we 
must  think  differently,  for  now  wre  know  that  everywhere  we 
have  to  deal  with  the  same  forces,  and  that  it  is  a  mere  question 
of  inventing  proper  methods  or  apparatus  for  rendering  them 
available. 

In  the  present  systems  oij  electrical  distribution,  the  employ- 
ment of  the  iron  with  its  wonderful  magnetic  properties  allows 
us  to  reduce  considerably  the  size  of  the  apparatus ;  but,  in  spite 
of  this,  it  is  still  very  cumbersome.  The  more  we  progress  in 
the  study  of  electric  and  magnetic  phenomena,  the  more  we  be- 


HIGH  FREQUENCY  AND  HIGH  POTENTIAL  CURRENTS.      193 

come  convinced  that  the  present  methods  will  be  short-lived.  For 
the  production  of  light,  at  least,  such  heavy  machinery  would 
seem  to  be  unnecessary.  The  energy  required  is  very  small,  and 
if  light  can  be  obtained  as  efficiently  as,  theoretically,  it  appears 
possible,  the  apparatus  need  have  but  a  very  small  output. 
There  being  a  strong  probability  that  the  illuminating  methods 
of  the  future  will  involve  the  use  of  very  high  potentials,  it  seems 
very  desirable  to  perfect  a  contrivance  capable  of  converting  the 
energy  of  heat  into  energy  of  the  requisite  form.  Nothing  to 
speak  of  has  been  done  towards  this  end,  for  the  thought  that 
electricity  of  some  50,000  or  100,000  volts  pressure  or  more,  even 
if  obtained,  would  be  unavailable  for  practical  purposes,  has  de- 
terred inventors  from  working  in  this  direction. 

In  Fig.  126  a  plan  of  connections  is  shown  for  converting 
currents  of  high,  into  currents  of  low,  tension  by  means  of  the 
disruptive  discharge  of  a  condenser.  This  plan  has  been  used  by 


FIG.  127. 

me  frequently  for  operating  a  few  incandescent  lamps  required 
in  the  laboratory.  Some  difficulties  have  been  encountered  in  the 
arc  of  the  discharge  which  I  have  been  able  to  overcome  to  a  great 
extent ;  besides  this,  and  the  adjustment  necessary  for  the  proper 
working,  no  other  difficulties  have  been  met  with,  and  it  was  easy 
to  operate  ordinary  lamps,  and  even  motors,  in  this  manner. 
The  line  being  connected  to  the  ground,  all  the  wires  could  be 
handled  with  perfect  impunity,  no  matter  how  high  the  potential 
at  the  terminals  of  the  condenser.  In  these  experiments  a  high 
tension  induction  coil,  operated  from  a  battery  or  from  an  alter- 
nate current  machine,  was  employed  to  charge  the  condenser  ;  but 
the  induction  coil  might  be  replaced  by  an  apparatus  of  a  differ- 
ent kind,  capable  of  giving  electricity  of  such  high  tension.  In 
this  manner,  direct  or  alternating  currents  may  be  converted,  and 
in  both  cases  the  current-impulses  may  be  of  any  desired  fre- 
quency. "When  the  currents  charging  the  condenser  are  of  the 


194  INVENTIONS  OF  NIKOLA  TE8LA. 

same  direction,  and  it  is  desired  that  the  converted  currents 
should  also  he  of  one  direction,  the  resistance  of  the  discharg- 
ing circuit  should,  of  course,  be  so  chosen  that  there  are  no 
oscillations. 

In  operating  devices  on  the  above  plan  I  have  observed  curi- 
ous phenomena  of  impedance  which  are  of  interest.  For  instance 
if  a  thick  copper  bar  be  bent,  as  indicated  in  Fig.  128,  and  shunted 
by  ordinary  incandescent  lamps,  then,  by  passing  the  discharge 
between  the  knobs,  the  lamps  may  be  brought  to  incandescence 
although  they  are  short-circuited.  When  a  large  induction  coil 


FIG.  128. 

is  employed  it  is  easy  to  obtain 
rendered  evident  by  the  different  degree  of  brilliancy  of  the 
lamps,  as  shown  roughly  in  Fig.  12S.  The  nodes  are  never  clearly 
delined,  but  they  are  simply  maxima  and  minima  of  potentials 
along  the  bar.  This  is  probably  due  to  the  irregularity  of  the  arc 
between  the  knobs.  In  general  when  the  above-described  plan 
of  conversion  from  high  to  low  tension  is  used,  the  behavior  of 
the  disruptive  discharge  may  be  closely  studied.  The  nodes  may 
also  be  investigated  by  means  of  an  ordinarv  Cardew  voltmeter 


HIGH  FREQUENCY  AND  HIGH  POTENTIAL  CURRENTS.      195 

which  should  be  well  insulated.  Geissler  tubes  may  also  be 
lighted  across  the  points  of  the  bent  bar ;  in  this  case,  of  course, 
it  is  better  to  employ  smaller  capacities.  I  have  found  it  prac- 
ticable to  light  up  in  this  manner  a  lamp,  and  even  a  Geissler 
tube,  shunted  by  a  short,  heavy  block  of  metal,  and  this  result 
seems  at  first  very  curious.  In  fact,  the  thicker  the  copper  bar 
in  Fig.  128,  the  better  it  is  for  the  success  of  the  experiments,  as 
they  appear  more  striking.  When  lamps  with  long  slender  fila- 
ments are  used  it  wTill  be  often  noted  that  the  filaments  are  from 
time  to  time  violently  vibrated,  the  vibration  being  smallest  at 
the  nodal  points.  This  vibration  seems  to  be  due  to  an  electro- 
static action  between  the  filament  and  the  glass  of  the  bulb. 

In  some  of  the  above  experiments  it  is  preferable  to  use  special 
lamps  having  a  straight  filament  as  shown  in  Fig.  129.  When 
such  a  lamp  is  used  a  still  more  curious  phenomenon  than  those 


Fro.  129. 

described  may  be  observed.  The  lamp  may  be  placed  across  the 
copper  bar  and  lighted,  and  by  using  somewhat  larger  capacities, 
or,  in  other  words,  smaller  frequencies  or  smaller  impulsive  im- 
pedances, the  filament  may  be  brought  to  any  desired  degree  of 
incandescence.  But  Avhen  the  impedance  is  increased,  a  point  is 
reached  when  comparatively  little  current  passes  through  the 
carbon,  and  most  of  it  through  the  rarefied  gas ;  or  perhaps  it 
may  be  more  correct  to  state  that  the  current  divides  nearly 
evenly  through  both,  in  spite  of  the  enormous  difference  in  the 
resistance,  and  this  would  be  true  unless  the  gas  and  the  filament 
behave  differently.  It  is  then  noted  that  the  whole  bulb  is  bril- 
liantly illuminated,  and  the  ends  of  the  leading-in  wires  become 
incandescent  and  often  throw  off  sparks  in  consequence  of  the 
violent  bombardment,  but  the  carbon  filament  remains  dark. 
This  is  illustrated  in  Fig.  129.  Instead  of  the  filament  a  single 


196  INVENTIONS  OF  NIKOLA  TESLA. 

wire  extending  through  the  whole  bulb  may  be  used,  and  in  this 
case  the  phenomenon  would  seem  to  be  still  more  interesting. 

From  the  above  experiment  it  will  be  evident,  that  when  ordi- 
nary lamps  are  operated  by  the  converted  currents,  those  should 
be  preferably  taken  in  which  the  platinum  wires  are  far  apart, 
and  the  frequencies  used  should  not  be  too  great,  else  the  dis- 
charge will  occur  at  the  ends  of  the  filament  or  in  the  base  of  the 
lamp  between  the  leading-in  wires,  and  the  lamp  might  then  be 
damaged. 

In  presenting  to  you  these  results  of  my  investigation  on  the 
subject  under  consideration,  I  have  paid  only  a  passing  notice  to 
facts  upon  which  I  could  have  dwelt  at  length,  and  among  many 
observations  I  have  selected  only  those  which  I  thought  most 
likely  to  interest  you.  The  field  is  wide  and  completely  unex- 
plored, and  at  every  step  a  new  truth  is  gleaned,  a  novel  fact 
observed. 

How  far  the  results  here  borne  out  are  capable  of  practical 
applications  will  be  decided  in  the  future.  As  regards  the  pro- 
duction of  light,  some  results  already  reached  are  encouraging 
and  make  me  confident  in  asserting  that  the  practical  solution  of 
the  problem  lies  in  the  direction  I  have  endeavored  to  indicate. 
Still,  whatever  may  be  the  immediate  outcome  of  these  experi- 
ments I  am  hopeful  that  they  will  only  prove  a  step  in  further 
development  towards  the  ideal  and  final  perfection.  The  possi- 
bilities which  are  opened  by  modern  research  are  so  vast  that 
even  the  most  reserved  must  feel  sanguine  of  the  future.  Emi- 
nent scientists  consider  the  problem  of  utilizing  one  kind  of 
radiation  without  the  others  a  rational  one.  In  an  apparatus  de- 
signed for  the  production  of  light  by  conversion  from  any  form 
of  energy  into  that  of  light,  such  a  result  can  never  be  reached, 
for  no  matter  what  the  process  of  producing  the  required  vibra- 
tions, be  it  electrical,  chemical  or  any  other,  it  will  not  be  possi- 
ble to  obtain  the  higher  light  vibrations  without  going  through 
the  lower  heat  vibrations.  It  is  the  problem  of  imparting  to  a 
body  a  certain  velocity  without  passing  through  all  lower  veloci- 
ties. But  there  is  a  possibility  of  obtaining  energy  not  only  in 
the  form  of  light,  but  motive  power,  and  energy  of  any  other 
form,  in  some  more  direct  way  from  the  medium.  The  time  will 
be  when  this  will  be  accomplished,  and  the  time  has  come  when 
one  may  utter  such  words  before  an  enlightened  audience  with- 
out being  considered  a  visionary.  AVe  are  whirling  through 


moil  FREQUENCY  AND  HIGH  POTENTIAL  CURRENTS.      197 

endless  space  with  an  inconceivable  speed,  all  around  us  every- 
thing is  spinning,  everything  is  moving,  everywhere  is  energy. 
There  must  be  some  way  of  availing  ourselves  of  this  energy 
more  directly.  Then,  with  the  light  obtained  from  the  medium, 
with  the  power  derived  from  it,  with  every  form  of  energy 
obtained  without  effort,  from  the  store  forever  inexhaustible, 
humanity  will  advance  with  giant  strides.  The  mere  contempla- 
tion of  these  magnificent  possibilities  expands  our  minds,  strength- 
ens our  hopes  and  fills  our  hearts  with  supreme  delight. 


CHAPTEE  XXVII. 

EXPERIMENTS  WITH  ALTERNATE  CURRENTS  OF  HIGH  POTENTIAL 

AND    HlGH    FREQUENCY.1 

I  CANNOT  find  words  to  express  how  deeply  I  feel  the  honor  of 
addressing  some  of  the  foremost  thinkers  of  the  present  time, 
and  so  many  able  scientific  men,  engineers  and  electricians,  of 
the  country  greatest  in  scientific  achievements. 

The  results  which  I  have  the  honor  to  present  before  such  a 
gathering  I  cannot  call  my  own.  There  are  among  you  not  a 
few  who  can  lay  better  claim  than  myself  on  any  feature  of 
merit  which  this  work  may  contain.  I  need  not  mention  many 
names  which  are  world-known — names  of  those  among  you  who 
are  recognized  as  the  leaders  in  this  enchanting  science ;  but  one, 
at  least,  I  must  mention — a  name  which  could  not  be  omitted  in 
a  demonstration  of  this  kind.  It  is  a  name  associated  with  the 
most  beautiful  invention  ever  made :  it  is  Crookes ! 

When  I  was  at  college,  a  good  while  ago,  I  read,  in  a  translation 
(for  then  I  was  not  familiar  with  your  magnificent  language),  the 
description  of  his  experiments  on  radiant  matter.  I  read  it  only 
once  in  my  life — that  time — yet  every  detail  about  that  charm- 
ing work  I  can  remember  to  this  day.  Few  are  the  books,  let  me 
say,  which  can  make  such  an  impression  upon  the  mind  of  a 
student. 

But  if,  on  the  present  occasion,  I  mention  this  name  as  one  of 
many  your  Institution  can  boast  of,  it  is  because  I  have  more 
than  one  reason  to  do  so.  For  what  I  have  to  tell  you  and  to 
show  you  this  evening  concerns,  in  a  large  measure,  that  same 
vague  world  which  Professor  Crookes  has  so  ably  explored ;  and, 
more  than  this,  when  I  trace  back  the  mental  process  which  led 
me  to  these  advances — which  even  by  myself  cannot  be  consid- 
ered trifling,  since  they  are  so  appreciated  by  you — I  believe 
that  their  real  origin,  that  which  started  me  to  work  in  this 

1.  Lecture  delivered  before  the  Institution  of  Electrical  Engineers,  London, 
February,  1892. 


HIGH  FREQUENCY  AND  HIGH  POTENTIAL  CURRENTS.      199 

direction,  and  brought  me  to  them,  after  a  long  period  of  con- 
stant thought,  was  that  fascinating  little  book  which  I  read  many 
years  ago. 

And  now  that  I  have  made  a  feeble  effort  to  express  my 
homage  and  acknowledge  my  indebedness  to  him  and  others 
among  you,  I  will  make  a  second  effort,  which  I  hope  you  will 
not  find  so  feeble  as  the  first,  to  entertain  you. 

Give  me  leave  to  introduce  the  subject  in  a  few  words. 

A  short  time  ago  I  had  the  honor  to  bring  before  our  Ameri- 
can Institute  of  Electrical  Engineers  some  results  then  arrived 
at  by  me  in  a  novel  line  of  work.  I  need  not  assure  you  that 
the  many  evidences  which  I  have  received  that  English  scientific 
men  and  engineers  were  interested  in  this  work  have  been  for 
me  a  great  reward  and  encouragement.  I  will  not  dwell  upon 
the  experiments  already  described,  except  with  the  view  of  com- 
pleting, or  more  clearly  expressing,  some  ideas^  advanced  by  me 
before,  and  also  with  the  view  of  rendering  the  study  here  pre- 
sented self-contained,  and  my  remarks  on  the  subject  of  this 
evening's  lecture  consistent. 

This  investigation,  then,  it  goes  without  saying,  deals  with 
alternating  currents,  and  to  be  more  precise,  with  alternating 
currents  of  high  potential  and  high  frequency.  Just  in  how 
much  a  very  high  frequency  is  essential  for  the  production  of 
the  results  presented  is  a  question  which,  even  with  my  present 
experience,  would  embarrass  me  to  answer.  Some  of  the  experi- 
ments may  be  performed  with  low  frequencies ;  but  very  high 
frequencies  are  desirable,  not  only  on  account  of  the  many  effects 
secured  by  their  use,  but  also  as  a  convenient  means  of  obtaining, 
in  the  induction  apparatus  employed,  the  high  potentials,  which  in 
their  turn  are  necessary  to  the  demonstration  of  most  of  the  ex- 
periments here  contemplated. 

Of  the  various  branches  of  electrical  investigation,  perhaps  the 
most  interesting  and  the  most  immediately  promising  is  that 
dealing  with  alternating  currents.  The  progress  in  this  branch 
of  applied  science  has  been  so  great  in  recent  years  that  it  justi- 
fies the  most  sanguine  hopes.  Hardly  have  we  become  familiar 
with  one  fact,  when  novel  .experiences  are  met  and  new  avenues 
of  research  are  opened.  Even  at  this  hour  possibilities  not 
dreamed  of  before  are,  by  the  use  of  these  currents,  partly  re- 
alized. As  in  nature  all  is  ebb  and  tide,  all  is  wave  motion,  so  it 
seems  that  in  all  branches  of  industry  alternating  currents — elec- 
tric wave  motion — will  have  the  sway. 


soo  INVENTIONS  OF  NIKOLA  TESLA. 

One  reason,  perhaps,  why  this  branch  of  science  is  being  so 
rapidly  developed  is  to  be  found  in  the  interest  which  is  attached 
to  its  experimental  study.  We  wind  a  simple  ring  of  iron  with 
coils ;  we  establish  the  connections  to  the  generator,  and  with 
wonder  and  delight  we  note  the  effects  of  strange  forces  which 
we  bring  into  play,  which  allow  us  to  transform,  to  transmit  and 
direct  energy  at  will.  We  arrange  the  circuits  properly,  and  we 
see  the  mass  of  iron  and  wires  behave  as  though  it  were  endowed 
with  life,  spinning  a  heavy  armature,  through  invisible  connec- 
tions, with  great  speed  and  power — with  the  energy  possibly  con- 
veyed from  a  great  distance.  We  observe  how  the  energy  of  an 
alternating  current  traversing  the  wire  manifests  itself — not  so 
much  in  the  wire  as  in  the  surrounding  space — in  the  most  sur- 
prising manner,  taking  the  forms  of  heat,  light,  mechanical 
energy,  and,  most  surprising  of  all,  even  chemical  affinity.  All 
these  observations  fascinate  us,  and  fill  us  with  an  intense  desire 
to  know  more  about  the  nature  of  these  phenomena.  Each  day 
we  go  to  our  work  in  the  hope  of  discovering, — in  the  hope  that 
some  one,  no  matter  who,  may  find  a  solution  of  one  of  the  pend- 
ing great  problems, — and  each  succeeding  day  we  return  to  our 
task  with  renewed  ardor ;  and  even  if  we  are  unsuccessful,  our 
work  has  not  been  in  vain,  for  in  these  strivings,  in  these  efforts, 
we  have  found  hours  of  untold  pleasure,  and  we  have  directed 
our  energies  to  the  benefit  of  mankind. 

We  may  take — at  random,  if  you  choose — any  of  the  many  ex- 
periments which  may  be  performed  with  alternating  currents ; 
a  few  of  which  only,  and  by  no  means  the  most  striking,  form 
the  subject  of  this  evening's  demonstration ;  they  are  all  equally 
interesting,  equally  inciting  to  thought. 

Here  is  a  simple  glass  tube  from  which  the  air  has  been  par- 
tially exhausted.  I  take  hold  of  it ;  I  bring  my  body  in  contact 
with  a  wire  conveying  alternating  currents  of  high  potential,  and 
the  tube  in  my  hand  is  brilliantly  lighted.  In  whatever  position 
I  may  put  it,  wherever  I  move  it  in  space,  as  far  as  I  can  reach, 
its  soft,  pleasing  light  persists  with  undiminished  brightness. 

Here  is  an  exhausted  bulb  suspended  from  a  single  wire. 
Standing  on  an  insulated  support,  I  grasp  it,  and  a  platinum  but- 
ton mounted  in  it  is  brought  to  vivid  incandescence. 

Here,  attached  to  a  leading  wire,  is  another  bulb,  which,  as  I 
touch  its  metallic  socket,  is  filled  with  magnificent  colors  of  phos- 
phorescent light. 


I 

HIGH  FREQUENCY  AND  HIGH  POTENTIAL  CURRENTS.      201 

Here  still  another,  which  by  my  fingers'  touch  casts  a  shadow 
— the  Crookes  shadow — of  the  stem  inside  of  it. 

Here,  again,  insulated  as  I  stand  on  this  platform,  I  bring  my 
body  in  contact  with  one  of  the  terminals  of  the  secondary  of 
this  induction  coil — with  the  end  of  a  wire  many  miles  long — and 
you  see  streams  of  light  break  forth  from  its  distant  end,  which 
is  set  in  violent  vibration. 

Here,  once  more,  I  attach  these  two  plates  of  wire  gauze  to  the 
terminals  of  the  coil ;  I  set  them  a  distance  apart,  and  I  set  the 
coil  to  work.  You  may  see  a  small  spark  pass  between  the 
plates.  I  insert  a  thick  plate  of  one  of  the  best  dielectrics  be- 
tween them,  and  instead  of  rendering  altogether  impossible,  as 
we  are  used  to  expect,  I  aid  the  passage  of  the  discharge,  which, 
as  I  insert  the  plate,  merely  changes  in  appearance  and  assumes 
the  form  of  luminous  streams. 

Is  there,  I  ask,  can  there  be,  a  more  interesting  study  than  that 
of  alternating  currents  ? 

In  all  these  investigations,  in  all  these  experiments,  which  are 
so  very,  very  interesting,  for  many  years  past — ever  since  the 
greatest  experimenter  who  lectured  in  this  hall  discovered  its 
principle — we  have  had  a  steady  companion,  an  appliance  familiar 
to  every  one,  a  plaything  once,  a  thing  of  momentous  importance 
now — the  induction  coil.  There  is  no  dearer  appliance  to  the 
electrician.  From  the  ablest  among  you,  I  dare  say,  down  to  the 
inexperienced  student,  to  your  lecturer,  we  all  have  passed  many 
delightful  hours  in  experimenting  with  the  induction  coil.  We 
have  watched  its  play,  and  thought  and  pondered  over  the  beau- 
tiful phenomena  which  it  disclosed  to  our  ravished  eyes.  So 
well  known  is  this  apparatus,  so  familiar  are  these  phenomena  to 
every  one,  that  my  courage  nearly  fails  me  when  I  think  that  I 
have  ventured  to  address  so  able  an  audience,  that  I  have  ven- 
tured to  entertain  you  with  that  same  old  subject.  Here  in  reality 
is  the  same  apparatus,  and  here  are  the  same  phenomena,  only 
the  apparatus  is  operated  somewhat  differently,  the  phenomena 
are  presented  in  a  different  aspect.  Some  of  the  results  we  find 
as  expected,  others  surprise  us,  but  all  captivate  our  attention,  for 
in  scientific  investigation  each  novel  result  achieved  may  be  the 
centre  of  a  new  departure,  each  novel  fact  learned  may  lead  to 
important  developments. 

Usually  in  operating  an  induction  coil  we  have  set  up  a  vibra- 
tion of  moderate  frequency  in  the  primary,  either  by  means  of  an 


203  INVENTIONS  OF  NIKOLA  TE8LA. 

interrupter  or  break,  or  by  the  use  of  an  alternator.  Earlier 
English  investigators,  to  mention  only  Spottiswoode  and  J.  E.  H. 
Gordon,  have  used  a  rapid  break  in  connection  with  the  coil. 
Our  knowledge  and  experience  of  to-day  enables  us  to  see  clearly 
why  these  coils  under  the  conditions  of  the  test  did  not  disclose 
any  remarkable  phenomena,  and  why  able  experimenters  failed 
to  perceive  many  of  the  curious  effects  which  have  since  been 
observed. 

In  the  experiments  such  as  performed  this  evening,  we  operate 
the  coil  either  from  a  specially  constructed  alternator  capable  of 
giving  many  thousands  of  reversals  of  current  per  second,  or,  by 
disrupt! vely  discharging  a  condenser  through  the  primary,  we  set 
up  a  vibration  in  the  secondary  circuit  of  a  frequency  of  many 
hundred  thousand  or  millions  per  second,  if  we  so  desire ;  and  in 
using  either  of  these  means  we  enter  a  field  as  yet  unexplored. 

It  is  impossible  to  pursue  an  investigation  in  any  novel  line 
without  finally  making  some  interesting  observation  or  learning 
some  useful  fact.  That  this  statement  is  applicable  to  the  sub- 
ject of  this  lecture  the  many  curious  and  unexpected  phenomena 
which  we  observe  afford  a  convincing  proof.  By  way  of  illustra- 
tion, take  for  instance  the  most  obvious  phenomena,  those  of  the 
discharge  of  the  induction  coil. 

Here  is  a  coil  which  is  operated  by  currents  vibrating  with 
extreme  rapidity,  obtained  by  disruptively  discharging  a  Leyden 
jar.  It  would  not  surprise  a  student  were  the  lecturer  to  say 
that  the  secondary  of  this  coil  consists  of  a  small  length  of  com- 
paratively stout  wire  ;  it  would  not  surprise  him  were  the  lecturer 
to  state  that,  in  spite  of  this,  the  coil  is  capable  of  giving  any 
potential  which  the  best  insulation  of  the  turns  is  able  to  with- 
stand ;  but  although  he  may  be  prepared,  and  even  be  indifferent 
as  to  the  anticipated  result,  yet  the  aspect  of  the  discharge  of  the 
coil  will  surprise  and  interest  him.  Every  one  is  familiar  with 
the  discharge  of  an  ordinary  coil ;  it  need  not  be  reproduced 
here.  But,  by  way  of  contrast,  here  is  a  form  of  discharge  of  a 
coil,  the  primary  current  of  which  is  vibrating  several  hundred 
thousand  times  per  second.  The  discharge  of  an  ordinary  coil 
appears  as  a  simple  line  or  band  of  light.  The  discharge  of  this 
coil  appears  in  the  form  of  powerful  brushes  and  luminous 
streams  issuing  from  all  points  of  the  two  straight  wires  attached 
to  the  terminals  of  the  secondary.  (Fig.  130.) 

compare  this  phenomenon  which  you  have  just  witnessed 


HIGH  FKEQ  UENCY  AND  HIGH  POTENTIAL  CURRENTS.      203 

with  the  discharge  of  a  Holtz  or  Wimshurst  machine — that  other 
interesting  appliance  so  dear  to  the  experimenter.  What  a  differ- 
ence there  is  between  these  phenomena !  And  yet,  had  I  made 
the  necessary  arrangements — which  could  have  been  made  easily, 
were  it  not  that  they  would  interfere  with  other  experiments — I 
could  have  produced  with  this  coil  sparks  which,  had  I  the  coil 


FIG.  131. 

hidden  from  your  view  and  only  two  knobs  exposed,  even  the 
keenest  observer  among  you  would  find  it  difficult,  if  not  impos- 
sible, to  distinguish  from  those  of  an  influence  or  friction  ma- 
chine. This  may  be  done  in  many  ways — for  instance,  by  oper- 
ating the  induction  coil  which  charges  the  condenser  from  an 
alternating-current  machine  of  very  low  frequency,  and  prefer- 
ably adjusting  the  discharge  circuit  so  that  there  are  no  oscillations 
set  up  in  it.  We  then  obtain  in  the  secondary  circuit,  if  the 
knobs  are  of  the  required  size  and  properly  set,  a  more  or  less 


204  INVENTIONS  OF  NIKOLA  TESLA. 

rapid  succession  of  sparks  of  great  intensity  and  small  quantity, 
which  possess  the  same  brilliancy,  and  are  accompanied  by  the 
same  sharp  crackling  sound,  as  those  obtained  from  a  friction  or 
influence  machine. 

Another  way  is  to  pass  through  two  primary  circuits,  having  a 
common  secondary,  two  currents  of  a  slightly  different  period, 
which  produce  in  the  secondary  circuit  sparks  occurring  at  com- 
paratively long  intervals.  But,  even  with  the  means  at  hand 
this  evening,  I  may  succeed  in  imitating  the  spark  of  a  Holtz 
machine.  For  this  purpose  I  establish  between  the  terminals  of 
the  coil  which  charges  the  condenser  a  long,  unsteady  arc,  which 
is  periodically  interrupted  by  the  upward  current  of  air  produced 
by  it.  To  increase  the  current  of  air  I  place  on  each  side  of  the 
arc,  and  close  to  it,  a  large  plate  of  mica.  The  condenser  charged 
from  this  coil  discharges  into  the  primary  circuit  of  a  second 
coil  through  a  small  air  gap,  which  is  necessary  to  produce  a 
sudden  rush  of  current  through  the  primary.  The  scheme  of 
connections  in  the  present  experiment  is  indicated  in  Fig.  131. 

G  is  an  ordinarily  constructed  alternator,  supplying  the  pri- 
mary P  of  an  induction  coil,  the  secondary  s  of  which  charges 
the  condensers  or  jars  c  c.  The  terminals  of  the  secondary  are 
connected  to  the  inside  coatings  of  the  jars,  the  outer  coatings 
being  connected  to  the  ends  of  the  primary  p  p  of  a  second  in- 
duction coil.  This  primary  p  p  has  a  small  air  gap  a  b. 

The  secondary  s  of  this  coil  is  provided  with  knobs  or  spheres 
K  K  of  the  proper  size  and  set  at  a  distance  suitable  for  the  ex- 
periment. 

•  A  long  arc  is  established  between  the  terminals  A  B  of  the  first 
induction  coil.  M  M  are  the  mica  plates. 

Each  time  the  arc  is  broken  between  A  and  B  the  jars  are 
quickly  charged  and  discharged  through  the  primary  p  p,  pro- 
ducing a  snapping  spark  between  the  knobs  K  K.  Upon  the  arc 
forming  between  A  and  B  the  potential  falls,  and  the  jars  cannot 
be  charged  to  such  high  potential  as  to  break  through  the  air 
gap  a  ~b  until  the  arc  is  again  broken  by  the  draught. 

In  this  manner  sudden  impulses,  at  long  intervals,  are  pro- 
duced in  the  primary  p  p,  which  in  the  secondary  s  give  a  cor- 
responding number  of  impulses  of  great  intensity.  If  the  sec- 
ondary knobs  or  spheres,  K  K,  are  of  the  proper  size,  the  sparks 
show  much  resemblance  to  those  of  a  Holtz  machine. 

But  these  two  effects,  which  to  the  e,ye  appear  so  very  differ- 


HIGH  FREQUENCY  AND  HIGH  POTENTIAL  CURRENTS.      205 

eut,  are  only  two  of  the  many  discharge  phenomena.  We  only 
need  to  change  the  conditions  of  the  test,  and  again  we  make 
other  observations  of  interest. 

When,  instead  of  operating  the  induction  coil  as  in  the  last 
two  experiments,  we  operate  it  from  a  high  frequency  alternator, 
as  in  the  next  experiment,  a  systematic  study  of  the  phenomena 
is  rendered  much  more  easy.  In  such  case,  in  varying  the 
strength  and  frequency  of  the  currents  through  the  primary,  we 
may  observe  live  distinct  forms  of  discharge,  which  I  have  de- 
scribed in  my  former  paper  on  the  subject  before  the  American 
Institute  of  Electrical  Engineers,  May  20,  1891. 

It  would  take  too  much  time,  and  it  would  lead  us  too  far 
from  the  subject  presented  this  evening,  to  reproduce  all  these 
forms,  but  it  seems  to  me  desirable  to  show  you  one  of  them.  It 
is  a  brush  discharge,  which  is  interesting  in  more  than  one  re- 
spect. Viewed  from  a  near  position  it  resembles  much  a  jet  of 
gas  escaping  under  great  pressure.  We  know  that  the  phenom- 
enon is  due  to  the  agitation  of  the  molecules  near  the  terminal, 
and  we  anticipate  that  some  heat  must  be  developed  by  the  im- 
pact of  the  molecules  against  the  terminal  or  against  each  other. 
Indeed,  we  find  that  the  brush  is  hot,  and  only  a  little  thought 
leads  us  to  the  conclusion  that,  could  we  but  reach  sufficiently 
high  frequencies,  we  could  produce  a  brush  which  would  give 
intense  light  and  heat,  and  which  would  resemble  in  every  par- 
ticular an  ordinary  flame,  save,  perhaps,  that  both  phenomena 
might  not  be  due  to  the  same  agent — save,  perhaps,  that  chemical 
affinity  might  not  be  electrical  in  its  nature. 

As  the  production  'of  heat  and  light  is  here  due  to  the  impact 
of  the  molecules,  or  atoms  of  air,  or  something  else  besides, 
and,  as  we  can  augment  the  energy  simply  by  raising  the 
potential,  we  might,  even  with  frequencies  obtained  from 
a  dynamo  machine,  intensify  the  action  to  such  a  degree  as  to 
bring  the  terminal  to  melting  heat.  But  with  such  low'  frequen- 
cies we  would  have  to  deal  always  with  something  of  the  nature 
of  an  electric  current.  If  I  approach  a  conducting  object  to  the 
brush,  a  thin  little  spark  passes,  yet,  even  with  the  frequencies 
used  this  evening,  the  tendency  to  spark  is  not  very  great.  So, 
for  instance,  if  I  hold  a  metallic  sphere  at  some  distance  above 
the  terminal,  you  may  see  the  whole  space  between  the  terminal 
and  sphere  illuminated  by  the  streams  without  the  spark  passing; 
and  with  the  much  higher  frequencies  obtainable  by  the  disrup- 


206  INVENTIONS  OF  NIKOLA  TESLA. 

tive  discharge  of  a  condenser,  were  it  not  for  the  sudden  impulses^ 
which  are  comparatively  few  in  number,  sparking  would  not 
occur  even  at  very  small  distances.  However,  with  incompar- 
ably higher  frequencies,  which  we  may  yet  lind  means  to  pro- 
duce efficiently,  and  provided  that  electric  impulses  of  such  high 
frequencies  could  be  transmitted  through  a  conductor,  the  elec- 
trical characteristics  of  the  brush  discharge  would  completely 
vanish — no  spark  would  pass,  no  shock  would  he  felt — yet  we 
would  still  have  to  deal  with  an  electric  phenomenon,  but  in  the 
broad,  modern  interpretation  of  the  word.  In  my  first  paper,  be- 
fore referred  to,  I  have  pointed  out  the  curious  properties  of  the 
brush,  and  described  the  best  manner  of  producing  it,  but  I  have 
thought  it  worth  while  to  endeavor  to  express  myself  more  clearly 
in  regard  to  this  phenomenon,  because  of  its  absorbing  interest. 

When  a  coil  is  operated  with  currents  of  very  high  freqency, 
beautiful  brush  effects  may  be  produced,  even  if  the  coil  be  of 
comparatively  small  dimensions.  The  experimenter  may  vary 
them  in  many  ways,  and,  if  it  were  for  nothing  else,  they  afford  a 
pleasing  sight.  What  adds  to  their  interest  is  that  they  may  be 
produced  with  one  single  terminal  as  well  as  with  two — in  fact, 
often  better  with  one  than  with  two. 

But  of  all  the  discharge  phenomena  observed,  the  most  pleas- 
ing to  the  eye,  and  the  most  instructive,  are  those  observed  with 
a  coil  which  is  operated  by  means  of  the  disruptive  discharge  of 
a  condenser.  The  power  of  the  brushes,  the  abundance  of  the 
sparks,  when  the  conditions  are  patiently  adjusted,  is  often  amaz- 
ing. With  even  a  very  small  coil,  if  it  be  so  well  insulated  as  to 
stand  a  difference  of  potential  of  several  thousand  volts  per  turn, 
the  sparks  may  be  so  abundant  that  the  whole  coil  may  appear 
a  complete  mass  of  fire. 

Curiously  enough  the  sparks,  when  the  terminals  of  the  coil 
are  set  at  a  considerable  distance,  seem  to  dart  in  every  possible 
direction  as  though  the  terminals  were  perfectly  independent  of 
each  other.  As  the  sparks  would  soon  destroy  the  insulation,  it 
is  necessary  to  prevent  them.  This  is  best  done  by  immersing 
the  coil  in  a  good  liquid  insulator,  such  as  boiled-out  oil.  Immer- 
sion in  a  liquid  may  be  considered  almost  an  absolute  necessity 
for  the  continued  and  successful  working  of  such  a  coil. 

It  is,  of  course,  out  of  the  question,  in  an  experimental  lecture, 
with  only  a  few  minutes  at  disposal  for  the  performance  of  each 
experiment,  to  show  these  discharge  phenomena  to  advantage, 


HIGH  FREQUENCY  AND  HIGH  POTENTIAL  CURRENTS.      207 

as,  to  produce  each  phenomenon  at  its  best,  a  very  careful  adjust- 
ment is  required.  But  even  if  imperfectly  produced,  as  they  are 
likely  to  be  this  evening,  they  are  sufficiently  striking  to  interest 
an  intelligent  audience. 

Before  showing  some  of  these  curious  effects  I  must,  for  the 
sake  of  completeness,  give  a  short  description  of  the  coil  and 
other  apparatus  used  in  the  experiments  with  the  disruptive  dis- 
charge this  evening. 

It  is  contained  in  a  box   u  (Fig.  13:2)  of  thick  boards  of  hard 


wood,  covered  on  the  outside  with  a  zinc  sheet  z,  which  is  carefully 
soldered  all  around.  It  might  be  advisable,  in  a  strictly  scientific 
investigation,  when  accuracy  is  of  great  importance,  to  do  away 
with  the  metal  cover,  as  it  might  introduce  many  errors,  princi- 
pally on  account  of  its  complex  action  upon  the  coil,  as  a  con- 
denser of  very  small  capacity  and  as  an  electrostatic  and  electro- 
magnetic screen.  When  the  coil  is  used  for  such  experiments  as 
are  here  contemplated,  the  employment  of  the  metal  cover  offers 
some  practical  advantages,  but  these  are  not  of  sufficient  import- 
ance to  be  dwelt  upon. 

The  coil  should  be  placed  symmetrically  to  the  metal  cover, 


308  INVENTIONS  OF  NIKOLA  TESLA. 

and  the  space  between  should,  of  course,  not  be  too  small,  cer- 
tainly not  less  than,  say,  five  centimetres,  but  much  more  if  pos- 
sible ;  especially  the  two  sides  of  the  zinc  box,  which  are  at  right 
angles  to  the  axis  of  the  coil,  should  be  sufficiently  remote  from 
the  latter,  as  otherwise  they  might  impair  its  action  and  be  a 
source  of  loss. 

The  coil  consists  of  two  spools  of  hard  rubber  R  K,  held  apart 
at  a  distance  of  10  centimetres  by  bolts  c  and  nuts  w,  likewise  of 
hard  rubber.  Each  spool  comprises  a  tube  T  of  approximately  8 
centimetres  inside  diameter,  and  3  millimetres  thick,  upon  which 
are  screwed  two  flanges  F  F,  24  centimetres  square,  the  space  be- 
tween the  flanges  being  about  3  centimetres.  The  secondary,  s  s, 
of  the  best  gutta  percha-covered  wire,  has  26  layers,  10  turns  in 
each,  giving  for  each  half  a  total  of  260  turns.  The  two  halves 
are  wound  oppositely  and  connected  in  series,  the  connection  be- 
tween both  being  made  over  the  primary.  This  disposition,  be- 
sides being  convenient,  has  the  advantage  that  when  the  coil  is 
well  balanced — that  is,  when  both  of  its  terminals  TJ,  T,,  are  con- 
nected to  bodies  or  devices  of  equal  capacity — there  is  not  much 
danger  of  breaking  through  to  the  primary,  and  the  insulation 
between  the  primary  and  the  secondary  need  not  be  thick.  In 
using  the  coil  it  is  advisable  to  attach  to  both  terminals  devices  of 
nearly  equal  capacity,  as,  when  the  capacity  of  the  terminals  is 
not  equal,  sparks  will  be  apt  to  pass  to  the  primary.  To  avoid 
this,  the  middle  point  of  the  secondary  may  be  connected  to  the 
primary,  but  this  is  not  always  practicable. 

The  primary  p  p  is  wound  in  two  parts,  and  oppositely,  upon 
a  wooden  spool  w,  and.the  four  ends  are  led  out  of  the  oil  through 
hard  rubber  tubes  t  t.  The  ends  of  the  secondary  Tt  Tt  are  also 
led  out  of  the  oil  through  rubber  tubes  t±  tv  of  great  thickness. 
The  primary  and  secondary  layers  are  insulated  by  cotton  cloth, 
the  thickness  of  the  .insulation,  of  course,  bearing  some  propor- 
tion to  the  difference  of  potential  between  the  turns  of  the  differ" 
ent  layers.  Each  half  of  the  primary  has  four  layers,  24  turns 
in  each,  this  giving  a  total  of  96  turns.  When  both  the  parts 
are  connected  in  series,  this  gives  a  ratio  of  conversion  of  about 
1 : 2.7,  and  with  the  primaries  in  multiple,  1  : 5.4 ;  but  in  operating 
with  very  rapidly  alternating  currents  this  ratio  does  not  convey 
even  an  approximate  idea  of  the  ratio  of  the  E.  M.  F'S.  in  the 
primary  and  secondary  circuits.  The  coil  is  held  in  position  in 
the  oil  on  wooden  supports,  there  being  about  5  centimetres 


HIGH  FRKQUENCY  AND  HIGH  POTENTIAL  CURRKNTti.      309 

thickness  of  oil  all  round.  Where  the  oil  is  not  specially  needed, 
the  space  is  filled  with  pieces  of  wood,  and  for  this  purpose 
principally  the  wooden  box  B  surrounding  the  whole  is  used. 

The  construction  here  shown  is,  of  course,  not  the  best  on 
general  principles,  but  I  believe  it  is  a  good  and  convenient  one 
for  the  production  of  effects  in  which  an  excessive  potential  and 
a  very  small  current  are  needed. 

In  connection  with  the  coil  I  use  either  the  ordinary  form  of* 
discharger  or  a  modified  form.  In  the  former  I  have  introduced 
two  changes  which  secure  some  advantages,  and  which  are  ob- 
vious. If  they  are  mentioned,  it  is  only  in  the  hope  that  some 
experimenter  may  find  them  of  use. 

One  of  the  changes  is  that  the  adjustable  knobs  A  and  B  (Fig. 
183),  of  the  discharger  are  held  in  jaws  of  brass,  .1  ,T,  by  spring 
pressure,  this  allowing  of  turning  them  successively  into  different 


FIG.  133. 

positions,  and  so  doing  away  with  the  tedious  process  of  frequent 
polishing  up. 

The  other  change  consists  in  the  employment  of  a  strong  elec- 
tromagnet N  s,  which  is  placed  with  its  axis  at  right  angles  to 
the  line  joining  the  knobs  A  and  B,  and  produces  a  strong  mag- 
netic field  between  them.  The  pole  pieces  of  the  magnet  are 
movable  and  properly  formed  so  as  to  protrude  between  the  brass 
knobs,  in  order  to  make  the  field  as  intense  as  possible;  but  to 
prevent  the  discharge  from  jumping  to  the  magnet  the  pole 
pieces  are  protected  by  a  layer  of  mica,  M  M,  of  sufficient  thick- 
ness; st  sl  and  ,92  .«?2  are  screws  for  fastening  the  wires.  On  each 
side  one  of  the  screws  is  for  large  and  the  other  for  small  wires. 
L  L  are  screws  for  fixing  in  position  the  rods  R  K,  which  support 
the  knobs. 


210  INVENTIONS  OF  NIKOLA  TESLA. 

In  another  arrangement  with  the  magnet  I  take  the  discharge 
between  the  rounded  pole  pieces  themselves,  which  in  such 
case  are  insulated  and  preferably  provided  with  polished  brass 
caps. 

The  employment  of  an  intense  magnetic  field  is  of  advantage 
principally  when  the  induction  coil  or  transformer  which  charges 
the  condenser  is  operated  by  currents  of  very  low  frequency.  In 
such  a  case  the  number  of  the  fundamental  discharges  between 
the  knobs  may  be  so  small  as  to  render  the  currents  produced  in 
the  secondary  unsuitable  for  many  experiments.  The  intense 
magnetic  field  then  serves  to  blow  out  the  arc  between  the  knobs 
as  soon  as  it  is  formed,  and  the  fundamental  discharges  occur  in 
quicker  succession. 

Instead  of  the  magnet,  a  draught  or  blast  of  air  may  be  em- 
ployed with  some  advantage.  In  this  case  the  arc  is  preferably 


FIG.  134. 

established  between  the  knobs  A  B,  in  Fig.  181  (the  knob-  "  l> 
being  generally  joined,  or  entirely  done  away  with),  as  in  this 
disposition  the  arc  is  long  and  unsteady,  and  is  easily  affected  by 
the  draught. 

When  a  magnet  is  employed  to  break  the  arc,  it  is  better  to 
choose  the  connection  indicated  diagrammatically  in  Fig.  134, 
as  in  this  case  the  currents  forming  the  arc  are  much  more  pow- 
erful, and  the  magnetic  field  exercises  a  greater  influence.  The 
use  of  the  magnet  permits,  however,  of  the  arc  being  replaced  by 
a  vacuum  tube,  but  I  have  encountered  great  difficulties  in  work- 
ing with  an  exhausted  tube. 

The  other  form  of  discharger  used  in  these  and  similar  experi- 
ments is  indicated  in  Figs.  135  and  13H.  It  consists  of  a  number 
of  brass  pieces  e  c  (Fig.  135),  each  of  which  comprises  a  spherical 
middle  portion  ///  with  an  extension  e  below — which  is  merely  used 
to  fasten  the  piece  in  a  lathe  when  polishing  up  the  discharging 


HIGH  FREQUENCY  AND  HIGH  POTENTIAL  CURRENTS.      211 

surface — and  a  column  above,  which  consists  of  a  knurled  flange 
f  surmounted  by  a  threaded  stem  I  carrying  a  nut  w,  by  means 
of  which  a  wire  is  fastened  to  the  column.  The  flange/  con- 
veniently serves  for  holding  the  brass  piece  when  fastening  the 


FIG.  135. 

wire,  and  also  for  turning  it  in  any  position  when  it  becomes 
necessary  to  present  a  fresh  discharging  surface.  Two  stout 
strips  of  hard  rubber  K  K,  with  planed  grooves  g  g  (Fig.  136)  to  fit 
the  middle  portion  of  the  pieces  c  c,  serve  to  clamp  the  latter 
and  hold  them  firmly  in  position  by  means  of  two  bolts  c  c 
(of  which  only  one  is  shown)  passing  through  the  ends  of  the 
strips. 

In  the  use  of  this  kind  of  discharger  I  have  found  three  prin- 
cipal advantages  over  the  ordinary  form.  First,  the  dielectric 
strength  of  a  given  total  widtli  of  air  space  is  greater  when  a 
great  many  small  air  gaps  are  used  instead  of  one,  which  permits 


FIG.  136. 

of  working  with  a  smaller  length  of  air  gap,  and  that  means 
smaller  loss  and  less  deterioration  of  the  metal;  secondly,  by 
reason  of  splitting  the  arc  up  into  smaller  arcs,  the  polished 
surfaces  are  made  to  last  much  longer;  and,  thirdly,  the  appa- 


212  INVENTIONS  OF  NIKOLA  TEftLA. 

ratus  affords  some  gauge  in  the  experiments.  I  usually  set  the 
pieces  by  putting  between  them  sheets  of  uniform  thickness  at  a 
certain  very  small  distance  which  is  known  from  the  experiments 
of  Sir  William  Thomson  to  require  a  certain  electromotive  force 
to  be  bridged  by  the  spark. 

It  should,  of  course,  be  remembered  that  the  sparking  distance 
is  much  diminished  as  the  frequency  is  increased.  By  taking 
any  number  of  spaces  the  experimenter  has  a  rough  idea  of  the 
electromotive  force,  and  he  finds  it  easier  to  repeat  an  experi- 
ment, as  he  has  not  the  trouble  of  setting  the  knobs  again  and 
again.  With  this  kind  of  discharger  I  have  been  able  to  main- 
tain an  oscillating  motion  without  any  spark  being  visible  with 
the  naked  eye  between  the  knobs,  and  they  would  not  show  a 
very  appeciable  rise  in  temperature.  This  form  of  discharge 
also  lends  itself  to  many  arrangements  of  condensers  and  circuits 
which  are  often  very  convenient  and  time-saving.  I  have  used 
it  preferably  in  a  disposition  similar  to  that  indicated  in  Fig.  131, 
when  the  currents  forming  the  arc  are  small. 

I  may  here  mention  that  I  have  also  used  dischargers  with 
single  or  multiple  air  gaps,  in  which  the  discharge  surfaces  were 
rotated  with  great  speed.  No  particular  advantage  was,  how- 
ever, gained  by  this  method,  except  in  cases  where  the  currents 
from  the  condenser  were  large  and  the  keeping  cool  of  the  sur- 
faces was  necessary,  and  in  cases  Avhen,  the  discharge  not  being 
oscillating  of  itself,  the  arc  as  soon  as  established  was  broken  by 
the  air  current,  thus  starting  the  vibration  at  intervals  in  rapid 
succession.  I  have  also  used  mechanical  interrupters  in  many 
ways.  To  avoid  the  difficulties  with  frictional  contacts,  the  pre- 
ferred plan  adopted  was  to  establish  the  arc  and  rotate  through 
it  at  great  speed  a  rim  of  mica  provided  with  many  holes  and 
fastened  to  a  steel  plate.  It  is  understood,  of  course,  that  the 
employment  of  a  magnet,  air  current,  or  other  interrupter,  pro- 
duces no  effect  worth  noticing,  unless  the  self-induction,  capacity 
and  resistance  are  so  related  that  there  are  oscillations  set  up 
upon  each  interruption. 

I  will  now  endeavor  to  show  you  some  of  the  most  noteworthy 
of  these  discharge  phenomena. 

I  have  stretched  across  the  room  two  ordinary  cotton  covered 
wires,  each  about  seven  metres  in  length.  They  are  supported 
011  insulating  cords  at  a  distance  of  about  thirty  centimetres.  I 
attach  now  to  each  of  the  terminals  of  the  coil  one  of  the  wires. 


HIGH  FREQUENCY  AND  HIGH  POTENTIAL  CURRENTS.      213 

and  set  the  coil  in  action.  Upon  turning  the  lights  off  in  the 
room  yon  see  the  wires  strongly  illuminated  by  the  streams  issu- 
ing abundantly  from  their  whole  surface  in  spite  of  the  cotton 
covering,  which  may  even  be  very  thick.  When  the  experiment 
is  performed  under  good  conditions,  the  light  from  the  wires  is 
sufficiently  intense  to  allow  distinguishing  the  objects  in  a  room. 
To  produce  the  best  result  it  is,  of  course,  necessary  to  adjust 
carefully  the  capacity  of  the  jars,  the  arc  between  the  knobs  and 
the  length  of  the  wires.  My  experience  is  that  calculation  of  the 
length  of  the  wires  leads,  in  such  case,  to  no  result  whatever.  The 
experimenter  will  do  best  to  take  the  wires  at  the  start  very  long, 
and  then  adjust  by  cutting  off  first  long  pieces,  and  then  smaller 
and  smaller  ones  as  he  approaches  the  right  length. 

A  convenient  way  is  to  use  an  oil  condenser  of  very  small 
capacity,  consisting  of  two  small  adjustable  metal  plates,  in  con- 
nection with  this  and  similar  experiments.  In  such  case  I  take 
wires  rather  short  and  at  the  beginning  set  the  condenser  plates 
at  maximum  distance.  If  the  streams  from  the  wires  increase  by 
approach  of  the  plates,  the  length  of  the  wires  is  about  right ;  if 
they  diminish,  the  wires  are  too  long  for  that  frequency  and  po- 
tential. When  a  condenser  is  used  in  connection  with  experi- 
ments with  such  a  coil,  it  should  be  an  oil  condenser  by  all  means, 
as  in  using  an  air  condenser  considerable  energy  might  be  wasted. 
The  wires  leading  to  the  plates  in  the  oil  should  be  very  thin, 
heavily  coated  with  some  insulating  compound,  and  provided 
with  a  conducting  covering — this  preferably  extending  under  the 
surface  of  the  oil.  The  conducting  cover  should  not  be  too  near 
the  terminals,  or  ends,  of  the  wire,  as  a  spark  would  be  apt  to 
jump  from  the  wire  to  it.  The  conducting  coating  is  used  to 
diminish  the  air  losses,  in  virtue  of  its  action  as  an  electrostatic 
screen.  As  to  the  size  of  the  vessel  containing  the  oil,  and  the 
size  of  the  plates,  the  experimenter  gains  at  once  an  idea  from  a 
rough  trial.  The  size  of  the  plates  in  oil  is,  however,  calculable, 
as  the  dielectric  losses  are  very  small. 

In  the  preceding  experiment  it  is  of  considerable  interest  to 
know  what  relation  the  quantity  of  the  light  emitted  bears  to 
the  frequency  and  potential  of  the  electric  impulses.  My  opinion 
is  that  the  heat  as  well  as  light  effects  produced  should  be  pro- 
portionate, under  otherwise  equal  conditions  of  test,  to  the  product 
of  frequency  and  square  of  potential,  but  the  experimental  veri- 
fication of  the  law,  whatever  it  may  be,  would  be  exceedingly 


214  INVENTIONS  OF  NIKOLA  TESLA. 

difficult.  One  thing  is  certain,  at  any  rate,  and  that  is,  that  in 
augmenting  the  potential  and  frequency  we  rapidly  intensify  the 
streams ;  and,  though  it  may  be  very  sanguine,  it  is  surely  not 
altogether  hopeless  to  expect  that  we  may  succeed  in  producing 
a  practical  illuminant  on  these  lines.  We  would  then  be  simply 
using  burners  or  flames,  in  which  there  would  be  no  chemical 
process,  no  consumption  of  material,  but  merely  a  transfer  of 
energy,  and  which  would,  in  all  probability,  emit  more  light  and 
less  heat  than  ordinary  flames. 

The  luminous  intensity  of  the  streams  is,  of  course,  considerably 


FIG.  137. 

increased  when  they  are  focused  upon  a  small  surface.  This  may 
be  shown  by  the  following  experiment : 

I  attach  to  one  of  the  terminals  of  the  coil  a  wire  w  (Fig.  137), 
bent  in  a  circle  of  about  30  centimetres  in  diameter,  and  to  the 
other  terminal  I  fasten  a  small  brass  sphere  s,  the  surface  of  the 
wire  being  preferably  equal  to  the  surface  of  the  sphere,  and  the 
centre  of  the  latter  being  in  a  line  at  right  angles  to  the  plane  of 
the  wire  circle  and  passing  through  its  centre.  When  the  dis- 
charge is  established  under  proper  conditions,  a  luminous  hollow 
cone  is  formed,  and  in  the  dark  one-half  of  the  brass  sphere  is 
strongly  illuminated,  as  shown  in  the  cut. 

By  some  artifice  or  other  it  is  easy  to  concentrate  the  streams 


HIGH  FREQUENCY  AND  HIGH  POTENTIAL  CURRENTS.      215 

upon  small  surfaces  and  to  produce  very  strong  light  effects. 
Two  thin  wires  may  thus  be  rendered  intensely  luminous. 

In  order  to  intensify  the  streams  the  wires  should  be  very  thin 
and  short ;  but  as  in  this  case  their  capacity  would  be  generally 
too  small  for  the  coil — at  least  for  such  a  one  as  the  present — it 
is  necessary  to  augment  the  capacity  to  the  required  value,  while, 
at  the  same  time,  the  surface  of  the  wires  remains  very  small. 
This  may  be  done  in  many  ways. 

Here,  for  instance,  I  have  two  plates,  K  K,  of  hard  rubber  (Fig. 
188),  upon  which  I  have  glued  two  very  thin  wires  w  w,  so  as  to 
form  a  name.  The  wires  may  be  bare  or  covered  with  the  best 
insulation — it  is  immaterial  for  the  success  of  the  experiment. 
Well  insulated  wires,  if  anything,  are  preferable.  On  the  back 


FIG.  138. 

of  each  plate,  indicated  by  the  shaded  portion,  is  a  tinfoil  coating 
t  t.  The  plates  are  placed  in  line  at  a  sufficient  distance  to  pre- 
vent a  spark  passing  from  one  wire  to  the  other.  The  two  tin- 
foil coatings  I  have  joined  by  a  conductor  c,  and  the  two  wires  I 
presently  connect  to  the  terminals  of  the  coil.  It  is  now  easy,  by 
varying  the  strength  and  frequency  of  the  currents  through  the 
primary,  to  find  a  point  at  which  the  capacity  of  the  system  is 
best  suited  to  the  conditions,  and  the  wires  become  so  strongly 
luminous  that,  when  the  light  in  the  room  is  turned  off  the  name 
formed  by  them  appears  in  brilliant  letters. 

It  is  perhaps  preferable  to  perform  this  experiment  with  a 
coil  operated  from  an  alternator  of  high  frequency,  as  then, 


216  INVENTIONS  OF  NIKOLA  TESLA. 

owing  to  the  harmonic  rise  and  fall,  the  streams  are  very  uniform, 
though  they  are  less  abundant  than  when  produced  with  such  a 
coil  as  the  present  one.  This  experiment,  however,  may  be  per- 
formed with  low  frequencies,  but  much  less  satisfactorily. 

When  two  wires,  attached  to  the  terminals  of  the  coil,  are  set 
at  the  proper  distance,  the  streams  between  them  may  be  so  in- 
tense as  to  produce  a  continuous  luminous  sheet.  To  show  this 
phenomenon  I  have  here  two  circles,  c  andc  (Fig.  139),  of  rather 
stout  wire,  one  being  about  80  centimetres  and  the  other  30  cen- 
timetres in  diameter.  To  each  of  the  terminals  of  the  coil  I 
attach  one  of  the  circles.  The  supporting  wires  are  so  bent  that 


FIG.  139. 

the  circles  may  be  placed  in  the  same  plane,  coinciding  as  nearly 
as  possible.  When  the  light  in  the  room  is  turned  off  and  the 
coil  set  to  work,  you  see  the  whole  space  between  the  wires  uni- 
formly filled  with  streams,  forming  a  luminous  disc,  which  could 
be  seen  from  a  considerable  distance,  such  is  the  intensity  of  the 
streams.  The  outer  circle  could  have  been  much  larger  than  the 
present  one;  in  fact,  with  this  coil  I  have  used  much  larger 
circles,  and  I  have  been  able  to  produce  a  strongly  luminous 
sheet,  covering  an  area  of  more  than  one  square  metre,  which  is 
a  remarkable  effect  with  this  very  small  coil.  To  avoid  uncer- 


HIGH  FREQUENCY  AND  HIGH  POTENTIAL  CURRENTS.      217 

tainty,  the  circle  has  been  taken  smaller,  and  the  area  is  now 
about  0.43  square  metre. 

The  frequency  of  the  vibration,  and  the  quickness  of  succes- 
sion of  the  sparks  between  the  knobs,  affect  to  a  marked  degree 
the  appearance  of  the  streams.  When  the  frequency  is  very 
low,  the  air  gives  way  in  more  or  less  the  same  manner,  as  by  a 
steady  difference  of  potential,  and  the  streams  consist  of  distinct 
threads,  generally  mingled  with  thin  sparks,  which  probably  cor- 
respond to  the  successive  discharges  occurring  between  the 
knobs.  But  when  the  frequency  is  extremely  high,  and  the  arc 
of  the  discharge  produces  a  very  loud  and  smooth  sound — show- 
ing both  that  oscillation  takes  place  and  that  the  sparks  succeed 
each  other  with  great  rapidity — then  the  luminous  streams 
formed  are  perfectly  uniform.  To  reach  this  result  very  small 
coils  and  jars  of  small  capacity  should  be  used.  I  take  two 
tubes  of  thick  Bohemian  glass,  about  5  centimetres  in  diameter 
and  20  centimetres  long.  In  each  of  the  tubes  I  slip  a  primary 
of  very  thick  copper  wire.  On  the  top  of  each  tube  I  wind  a 
secondary  of  much  thinner  gutta-percha  covered  wire.  The  two 
secondaries  I  connect  in  series,  the  primaries  preferably  in  multiple 
arc.  The  tubes  are  then  placed  in  a  large  glass  vessel,  at  a  dis- 
tance of  10  to  15  centimetres  from  each  other,  on  insulating  sup- 
ports, and  the  vessel  is  filled  witli  boiled-out  oil,  the  oil  reaching 
about  an  inch  above  the  tubes.  The  free  ends  of  the  secondary 
are  lifted  out  of  the  coil  and  placed  parallel  to  each  other  at  a 
distance  of  about  ten  centimetres.  The  ends  which  are  scraped 
should  be  dipped  in  the  oil.  Two  four-pint  jars  joined  in  series 
may  be  used  to  discharge  through  the  primary.  When  the  ne- 
cessary adjustments  in  the  length  and  distance  of  the  wires  above 
the  oil  and  in  the  arc  of  discharge  are  made,  a  luminous  sheet  is 
produced  between  the  wires  which  is  perfectly  smooth  and  tex- 
tureless,  like  the  ordinary  discharge  through  a  moderately  ex- 
hausted tube. 

I  have  purposely  dwelt  upon  this  apparently  insignificant  ex- 
periment. In  trials  of  this  kind  the  experimenter  arrives  at  the 
startling  conclusion  that,  to  pass  ordinary  luminous  discharges 
through  gases,  no  particular  degree  of  exhaustion  is  needed,  but 
that  the  gas  may  be  at  ordinary  or  even  greater  pressure.  To 
accomplish  this,  a  very  high  frequency  is  essential ;  a  high  po- 
tential is  likewise  required,  but  this  is  merely  an  incidental  neces- 
sity. These  experiments  teach  us  that,  in  endeavoring  to  dis- 


218  INVENTIONS  OF  NIKOLA   TE8LA. 

cover  novel  methods  of  producing  light  by  the  agitation  of  atoms, 
or  molecules,  of  a  gas,  we  need  not  limit  our  research  to  the 
vacuum  tube,  but  may  look  forward  quite  seriously  to  the  possi- 
bility of  obtaining  the  light  effects  without  the  use  of  any  vessel 
whatever,  with  air  at  ordinary  pressure. 

Such  discharges  of  very  high  frequency,  which  render  luminous 
the  air  at  ordinary  pressures,  we  have  probably  occasion  often  to 
witness  in  Nature.  I  have  no  doubt  that  if,  as  many  believe,  the 
aurora  borealis  is  produced  by  sudden  cosmic  disturbances,  such 
as  eruptions  at  the  sun's  surface,  which  set  the  electrostatic  charge 
of  the  earth  in  an  extremely  rapid  vibration,  the  red  glow  ob- 
served is  not  confined  to  the  upper  rarefied  strata  of  the  air,  but 
the  discharge  traverses,  by  reason  of  its  very  high  frequency, 
also  the  dense  atmosphere  in  the  form  of  a  glow,  such  as  we  or- 
dinarily produce  in  a  slightly  exhausted  tube.  If  the  frequency 
were  very  low,  or  even  more  so,  if  the  charge  were  not  at  all 
vibrating,  the  dense  air  would  break  down  as  in  a  lightning  dis- 
charge. Indications  of  such  breaking  down  of  the  lower  dense 
strata  of  the  air  have  been  repeatedly  observed  at  the  occurence 
of  this  marvelous  phenomenon  ;  but  if  it  does  occur,  it  can  only 
be  attributed  to  the  fundamental  disturbances,  which  are  few  in 
number,  for  the  vibration  produced  by  them  would  be  far  too 
rapid  to  allow  a  disruptive  break.  It  is  the  original  and  irregular 
impulses  which  affect  the  instruments ;  the  superimposed  vibra- 
tions probably  pass  unnoticed. 

When  an  ordinary  low  frequency  discharge  is  passed  through 
moderately  rarefied  air,  the  air  assumes  a  purplish  hue.  If  by 
some  means  or  other  we  increase  the  intensity  of  the  molecular, 
or  atomic,  vibration,  the  gas  changes  to  a  white  color.  A  similar 
change  occurs  at  ordinary  pressures  with  electric  impulses  of  very 
high  frequency.  If  the  molecules  of  the  air  around  a  wire  are 
moderately  agitated,  the  brush  formed  is  reddish  or  violet ;  if 
the  vibration  is  rendered  sufficiently  intense,  the  streams  become 
white.  We  may  accomplish  this  in  various  ways.  In  the  experi- 
ment before  shown  with  the  two  wires  across  the  room,  I  have 
endeavored  to  secure  the  result  by  pushing  to  a  high  value  both 
the  frequency  and  potential ;  in  the  experiment  with  the  thin 
wires  glued  on  the  rubber  plate  I  have  concentrated  the  action 
upon  a  very  small  surface — in  other  words,  I  have  worked  with 
a  great  electric  density. 

A  most  curious  form  of  discharge  is  observed  with  such  a  coil 


man  FREQUENCY  AND  man  POTENTIAL  CURRENTS.    219 

when  the  frequency  and  potential  are  pushed  to  the  extreme 
limit.  To  perform  the  experiment,  every  part  of  the  coil  should 
be  heavily  insulated,  and  only  two  small  spheres — or,  better  still, 
two  sharp-edged  metal  discs  (d  d,  Fig.  140)  of  no  more  than 
a  few  centimetres  in  diameter — should  be  exposed  to  the  air. 
The  coil  here  used  is  immersed  in  oil,  and  the  ends  of  the 
secondary  reaching  out  of  the  oil  are  covered  with  an  air-tight 
cover  of  hard  rubber  of  great  thickness.  All  cracks,  if  there 
are  any,  should  be  carefully  stopped  up,  so  that  the  brush  dis- 
charge cannot  form  anywhere  except  on  the  small  spheres  or 
plates  which  are  exposed  to  the  air.  In  this  case,  since  there 
are  no  large  plates  or  other  bodies  of  capacity  attached  to  the 
terminals,  the  coil  is  capable  of  an  extremely  rapid  vibration. 


FIG.  140. 

The  potential  may  be  raised  by  increasing,  as  far  as  the  experi- 
menter judges  proper,  the  rate  of  change  of  the  primary  cur- 
rent. With  a  coil  not  widely  differing  from  the  present,  it  is 
best  to  connect  the  two  primaries  in  multiple  arc ;  but  if  the 
secondary  should  have  a  much  greater  number  of  turns  the 
primaries  should  preferably  be  used  in  series,  as  otherwise  the 
vibration  might  be  too  fast  for  the  secondary.  It  occurs  under 
these  conditions  that  misty  white  streams  break  forth  from  the 
edges  of  the  discs  and  spread  out  phantom-like  into  space. 
With  this  coil,  when  fairly  well  produced,  they  are  about  25  to 
30  centimetres  long.  When  the  hand  is  held  against  them  no 
sensation  is  produced,  and  a  spark,  causing  a  shock,  jumps  from 


220  INVENTIONS  OF  NIKOLA  TESLA. 

the  terminal  only  upon  the  hand  being  brought  much  nearer. 
If  the  oscillation  of  the  primary  current  is  rendered  intermittent 
by  some  means  or  other,  there  is  a  corresponding  throbbing  of 
the  streams,  and  now  the  hand  or  other  conducting  object  may 
be  brought  in  still  greater  proximity  to  the  terminal  without  a 
spark  being  caused  to  jump. 

Among  the  many  beautiful  phenomena  which  may  be  pro- 
duced with  such  a  coil,  I  have  here  selected  only  those  which  ap- 
pear to  possess  some  features  of  novelty,  and  lead  us  to  some 
conclusions  of  interest.  One  will  not  tind  it  at  all  difficult  to 
produce  in  the  laboratory,  by  means  of  it,  many  other  phenomena 
which  appeal  to  the  eye  even  more  than  these  here  shown,  but 
present  no  particular  feature  of  novelty. 

Early  experimenters  describe  the  display  of  sparks  produced  by 
an  ordinary  large  induction  coil  upon  an  insulating  plate  separat- 
ing the  terminals.  Quite  recently  Siemens  performed  some  ex- 
periments in  which  fine  effects  were  obtained,  which  were  seen 
by  many  with  interest.  No  doubt  large  coils,  even  if  operated 
with  currents  of  low  frequencies,  are  capable  of  producing 
beautiful  effects.  But  the  largest  coil  ever  made  could  not,  by 
far,  equal  the  magnificent  display  of  streams  and  sparks  obtained 
from  such  a  disruptive  discharge  coil  when  properly  adjusted. 
To  give  an  idea,  a  coil  such  as  the  present  one  will  cover  easily 
a  plate  of  one  metre  in  diameter  completely  with  the  streams. 
The  best  way  to  perform  such  experiments  is  to  take  a  very  thin 
rubber  or  a  glass  plate  and  glue  on  one  side  of  it  a  narrow  ring 
of  tinfoil  of  very  large  diameter,  and  on  the  other  a  circular 
washer,  the  centre  of  the  latter  coinciding  with  that  of  the  ring, 
and  the  surfaces  of  both  being  preferably  equal,  so  as  to  keep 
the  coil  well  balanced.  The  washer  and  ring  should  be  connected 
to  the  terminals  by  heavily  insulated  thin  wires.  It  is  easy  in 
observing  the  effect  of  the  capacity  to  produce  a  sheet  of  uni- 
form streams,  or  a  fine  network  of  thin  silvery  threads,  or  a 
mass  of  loud  brilliant  sparks,  which  completely  cover  the  plate. 

Since  I  have  advanced  the  idea  of  the  conversion  by  means  of 
the  disruptive  discharge,  in  my  paper  before  the  American  In- 
stitute of  Electrical  Engineers  at  the  beginning  of  the  past  year, 
the  interest  excited  in  it  has  been  considerable.  It  affords  us  a 
means  for  producing  any  potentials  by  the  aid  of  inexpensive 
coils  operated  from  ordinary  systems  of  distribution,  and — what 
is  perhaps  more  appreciated — it  enables  us  to  convert  cunvnt*  <>!' 


HIGH  FliKQ.UENCY  AND  HIGH  POTENTIAL  CURRENTS.      231 

any  frequency  into  currents  of  any  other  lower  or  higher  fre- 
quency. But  its  chief  value  will  perhaps  be  found  in  the  help 
which  it  will  afford  us  in  the  investigations  of  the  phenomena 
of  phosphorescence,  which  a  disruptive  discharge  coil  is  capable 
of  exciting  in  innumerable  cases  where  ordinary  coils,  even  the 
largest,  would  utterly  fail. 

Considering  its  probable  uses  for  many  practical  purposes,  and 
its  possible  introduction  into  laboratories  for  scientific  research, 
a  few  additional  remarks  as  to  the  construction  of  such  a  coil 
will  perhaps  not  be  found  superfluous. 

It  is,  of  course,  absolutely  necessary  to  employ  in  such  a  coil 
wires  provided  with  the  best  insulation. 

Good  coils  may  be  produced  by  employing  wires  covered  with 
several  layers  of  cotton,  boiling  the  coil  a  long  time  in  pure  wax, 
and  cooling  under  moderate  pressure.  The  advantage  of  such  a 
coil  is  that  it  can  be  easily  handled,  but  it  cannot  probably  give 
as  satisfactory  results  as  a  coil  immersed  in  pure  oil.  Besides,  it 
seems  that  the  presence  of  a  large  body  of  wax  affects  the  coil 
disadvantageously,  whereas  this  does  not  seem  to  be  the  case  with 
oil.  Perhaps  it  is  because  the  dielectric  losses  in  the  liquid  are 
smaller. 

I  have  tried  at  iirst  silk  and  cotton  covered  wires  with  oil  im- 
mersions, but  I  have  been  gradually  led  to  use  gutta-percha 
covered  wires,  which  proved  most  satisfactory.  Gutta-percha 
insulation  adds,  of  course,  to  the  capacity  of  the  coil,  and  this, 
especially  if  the  coil  be  large,  is  a  great  disadvantage  when  ex- 
treme frequencies  are  desired ;  but,  on  the  other  hand,  gutta- 
percha  will  withstand  much  more  than  an  equal  thickness  of  oil, 
and  this  advantage  should  be  secured  at  any  price.  Once  the 
coil  has  been  immersed,  it  should  never  be  taken  out  of  the  oil 
for  more  than  a  few  hours,  else  the  gutta-percha  will  crack  up 
and  the  coil  will  not  be  worth  half  as  much  as  before.  Gutta- 
percha  is  probably  slowly  attacked  by  the  oil,  but  after  an  im- 
mersion of  eight  to  nine  months  I  have  found  no  ill  effects. 

I  have  obtained  two  kinds  of  gutta-percha  wire  known  in  com- 
merce :  in  one  the  insulation  sticks  tightly  to  the  metal,  in  the 
other  it  does  not.  Unless  a  special  method  is  followed  to  expel  all 
air,  it  is  much  safer  to  use  the  iirst  kind.  I  wind  the  coil  within 
an  oil  tank  so  that  all  interstices  are  filled  up  with  the  oil.  Be- 
tween the  layers  I  use  cloth  boiled  out  thoroughly  in  oil, 
calculating  the  thickness  according  to  the  difference  of  potential 


222  INVENTIONS  OF  NIKOLA  TESLA. 

between  the  turns.  There  seems  not  to  be  a  very  great  differ- 
ence whatever  kind  of  oil  is  used ;  I  use  paraffine  or  linseed  oil. 

To  exclude  more  perfectly  the  air,  an  excellent  way  to  pro- 
ceed, and  easily  practicable  with  small  coils,  is  the  following : 
Construct  a  box  of  hardwood  of  very  thick  boards  which  have 
been  for  a  long  time  boiled  in  oil.  The  boards  should  be  so 
joined  as  to  safely  withstand  the  external  air  pressure.  The  coil 
being  placed  and  fastened  in  position  within  the  box,  the  latter 
is  closed  with  a  strong  lid,  and  covered  with  closely  fitting  metal 
sheets,  the  joints  of  which  are  soldered  very  carefully.  On  the 
top  two  small  holes  are  drilled,  passing  through  the  metal  sheet 
and  the  wood,  and  in  these  holes  two  small  glass  tubes  are  insert- 
ed and  the  joints  made  air-tight.  One  of  the  tubes  is  connected 
to  a  vacuum  pump,  and  the  other  with  a  vessel  containing  a 
sufficient  quantity  of  boiled-out  oil.  The  latter  tube  has  a  very 
small  hole  at  the  bottom,  and  is  provided  with  a  stopcock. 
When  a  fairly  good  vacuum  has  been  obtained,  the  stopcock  is 
opened  and  the  oil  slowly  fed  in.  Proceeding  in  this  manner, 
it  is  impossible  that  any  big  bubbles,  which  are  the  principal 
danger,  should  remain  between  the  turns.  The  air  is  most  com- 
pletely excluded,  probably  better  than  by  boiling  out,  which, 
however,  when  gutta-percha  coated  wires  are  used,  is  not  prac- 
ticable. 

For  the  primaries  I  use  ordinary  line  wire  with  a  thick  cotton 
coating.  Strands  of  very  thin  insulated  wires  properly  inter- 
laced would,  of  course,  be  the  best  to  employ  for  the  primaries, 
but  they  are  not  to  be  had. 

In  an  experimental  coil  the  size  of  the  wires  is  not  of  great 
importance.  In  the  coil  here  used  the  primary  is  No.  12  and  the 
secondary  No.  24  Brown  &  Sharpe  gauge  wire ;  but  the  sections 
may  be  varied  considerably.  It  would  only  imply  different  ad- 
justments ;  the  results  aimed  at  would  not  be  materially  affected. 

I  have  dwelt  at  some  length  upon  the  various  forms  of  brush 
discharge  because,  in  studying  them,  we  not  only  observe  pheno- 
mena which  please  our  eye,  but  also  afford  us  food  for  thought, 
and  lead  us  to  conclusions  of  practical  importance.  In  the  use 
of  alternating  currents  of  very  high  tension,  too  much  precaution 
cannot  be  taken  to  prevent  the  brush  discharge.  In  a  main  con- 
veying such  currents,  in  an  induction  coil  or  transformer,  or  in  a 
condenser,  the  brush  discharge  is  a  source  of  great  danger  to  the 
insulation.  In  a  condenser,  especially,  the  gaseous  matter  must 


HIGH  FREQUENCY  AND  HIGH  POTENTIAL  CURRENTS.     223 

be  most  carefully  expelled,  for  in  it  the  charged  surfaces  are  near 
each  other,  and  if  the  potentials  are  high,  just  assure  as  a  weight 
will  fall  if  let  go,  so  the  insulation  will  give  way  if  a  single 
gaseous  bubble  of  some  size  be  present,  whereas,  if  all  gaseous 
matter  were  carefully  excluded,  the  condenser  would  safely 
withstand  a  much  higher  difference  of  potential.  A  main  con- 
veying alternating  currents  of  very  high  tension  may  be  injured 
merely  by  a  blow  hole  or  small  crack  in  the  insulation,  the  more 
so  as  a  blowhole  is  apt  to  contain  gas  at  low  pressure  ;  and  as  it 
appears  almost  impossible  to  completely  obviate  such  little  im- 
perfections, I  am  led  to  believe  that  in  our  future  distribution  of 
electrical  energy  by  currents  of  very  high  tension,  liquid  insula- 
tion will  be  used.  The  cost  is  a  great  drawback,  but  if  we  em- 
ploy an  oil  as  an  insulator  the  distribution  of  electrical  energy 
with  something  like  100,000  volts,  and  even  more,  becomes,  at 
least  with  higher  frequencies,  so  easy  that  it  could  be  hardly 
called  an  engineering  feat.  With  oil  insulation  and  alternate  cur- 
rent motors,  transmissions  of  power  can  be  affected  with  safety 
and  upon  an  industrial  basis  at  distances  of  as  much  as  a  thousand 
miles. 

A  peculiar  property  of  oils,  and  liquid  insulation  in  general, 
when  subjected  to  rapidly  changing  electric  stresses,  is  to  disperse 
any  gaseous  bubbles  which  may  be  present,  and  diffuse  them 
through  its  mass,  generally  long  before  any  injurious  break  can 
occur.  This  feature  may  be  easily  observed  with  an  ordinary  in- 
duction coil  by  taking  the  primary  out,  plugging  up  the  end  of 
the  tube  upon  which  the  secondary  is  wound,  and  filling  it  with 
some  fairly  transparent  insulator,  such  as  paraffme  oil.  A  prim- 
ary of  a  diameter  something  like  six  millimetres  smaller  than  the 
inside  of  the  tube  may  be  inserted  in  the  oil.  When  the  coil  is 
set  to  work  one  may  see,  looking  from  the  top  through  the  oil, 
many  luminous  points — air  bubbles  which  are  caught  by  insert- 
ing the  primary,  and  which  are  rendered  luminous  in  consequence 
of  the  violent  bombardment.  The  occluded  air,  by  its  impact 
against  the  oil,  heats  it ;  the  oil  begins  to  circulate,  carrying  some 
of  the  air  along  with  it,  until  the  bubbles  are  dispersed  and  the 
luminous  points  disappear.  In  this  manner,  unless  large  bubbles 
are  occluded  in  such  way  that  circulation  is  rendered  impossible, 
a  damaging  break  is  averted,  the  only  effect  being  a  moderate 
warming  up  of  the  oil.  If,  instead  of  the  liquid,  a  solid  insula- 
tion, no  matter  how  thick,  were  used,  a  breaking  through  and  in- 
jury of  the  apparatus  would  be  inevitable. 


234  INVENTIONS  OF  NIKOLA   TKSLA. 

The  exclusion  of  gaseous  matter  from  any  apparatus  in  which 
the  dielectric  is  subjected  to  more  or  less  rapidly  changing  elec- 
tric forces  is,  however,  not  only  desirable  in  order  to  avoid  a 
possible  injury  of  the  apparatus,  but  also  on  account  of  economy. 
In  a  condenser,  for  instance,  as  long  as  only  a  solid  or  only  a 
liquid  dielectric  is  used,  the  loss  is  small ;  but  if  a  gas  under  or- 
dinary or  small  pressure  be  present  the  loss  may  be  very  great. 
Whatever  the  nature  of  the  force  acting  in  the  dielectric  may  be, 
it  seems  that  in  a  solid  or  liquid  the  molecular  displacement  pro- 
duced by  the  force  is  small :  hence  the  product  of  force  and 
displacement  is  insignificant,  unless  the  force  be  very  great ;  but 
in  a  gas  the  displacement,  and  therefore  this  product,  is  consider- 
able ;  the  molecules  are  free  to  move,  they  reach  high  speeds,  and 
the  energy  of  their  impact  is  lost  in  heat  or  otherwise.  If  the 
gas  be  strongly  compressed,  the  displacement  due  to  the  force  is 
made  smaller,  and  the  losses  are  reduced. 

In  most  of  the  succeeding  experiments  I  prefer,  chiefly  on 
account  of  the  regular  and  positive  action,  to  employ  the  alter- 
nator before  referred  to.  This  is  one  of  the  several  machines 
constructed  by  me  for  the  purpose  of  these  investigations.  It  has 
384  pole  projections,  and  is  capable  of  giving  currents  of  a  fre- 
quency of  about  10,000  per  second.  This  machine  has  been  illus- 
trated and  briefly  described  in  my  first  paper  before  the  American 
Institute  of  Electrical  Engineers,  May  20th,  1891,  to  which  I  have 
already  referred.  A  more  detailed  description,  sufficient  to  en- 
able any  engineer  to  build  a  similar  machine,  will  be  found  in 
several  electrical  journals  of  that  period. 

The  induction  coils  operated  from  the  machine  are  rather  small, 
containing  from  5,000  to  15,000  turns  in  the  secondary.  They 
are  immersed  in  boiled-out  linseed  oil,  contained  in  wooden  boxes 
covered  with  zinc  sheet. 

I  have  found  it  advantageous  to  reverse  the  usual  position  of 
the  wires,  and  to  wind,  in  these  coils,  the  primaries  on  the  top ; 
thus  allowing  the  use  of  a  much  larger  primary,  which,  of  course, 
reduces  the  danger  of  overheating  and  increases  the  output  of 
the  coil.  I  make  the  primary  on  each  side  at  least  one  centimetre 
shorter  than  the  secondary,  to  prevent  the  breaking  through  on  the 
ends,  which  would  surely  occur  unless  the  insulation  on  the  top 
of  the  secondary  be  very  thick,  and  this,  of  course,  would  be  dis- 
advantageous. 

When  the  primary  is  made   movable,   which  is  necessary  in 


HIGH  FREQUENCY  AND  HIGH  POTENTIAL  CURRENTS.     225 

some  experiments,  and  many  times  convenient  for  the  purposes 
of  adjustment,  I  cover  the  secondary  with  wax,  and  turn  it  off 
in  a  lathe  to  a  diameter  slightly  smaller  than  the  inside  of  the 
primary  coil.  The  latter  I  provide  with  a  handle  reaching  out 
of  the  oil,  which  serves  to  shift  it  in  any  position  along  the 
secondary. 

I  will  now  venture  to  make,  in  regard  to  the  general  mani- 
pulation of  induction  coils,  a  few  observations  bearing  upon  points 
which  have  not  been  fully  appreciated  in  earlier  experiments 
with  such  coils,  and  are  even  now  often  overlooked. 

The  secondary  of  the  coil  possesses  usually  such  a  high  self- 
induction  that  the  current  through  the  wire  is  inappreciable,  and 
may  be  so  even  when  the  terminals  are  joined  by  a  conductor  of 
small  resistance.  If  capacity  is  added  to  the  terminals,  the  self- 
induction  is  counteracted,  and  a  stronger  current  is  made  to  flow 
through  the  secondary,  though  its  terminals  are  insulated  from 
each  other.  To  one  entirely  unacquainted  with  the  properties  of 
alternating  currents  nothing  will  look  more  puzzling.  This  fea- 
ture was  illustrated  in  the  experiment  performed  at  the  beginning 
with  the  top  plates  of  wire  gauze  attached  to  the  terminals  and 
the  rubber  plate.  When  the  plates  of  wire  gauze  were  close  to- 
gether, and  a  small  arc  passed  between  them,  the  arc  prevented  a 
strong  current  from  passing  through  the  secondary,  because  it 
did  away  with  the  capacity  on  the  terminals ;  when  the  rubber 
plate  was  inserted  between,  the  capacity  of  the  condenser  formed 
counteracted  the  self-induction  of  the  secondary,  a  stronger  cur- 
rent passed  now,  the  coil  performed  more  work,  and  the  discharge 
was  by  far  more  powerful. 

The  first  thing,  then,  in  operating  the  induction  coil  is  to  com- 
bine capacity  with  the  secondary  to  overcome  the  self-induction. 
If  the  frequencies  and  potentials  are  very  high,  gaseous  matter 
should  be  carefully  kept  away  from  the  charged  surfaces.  If 
Leyden  jars  are  used,  they  should  be  immersed  in  oil,  as  other- 
wise considerable  dissipation  may  occur  if  the  jars  are  greatly 
strained.  When  high  frequencies  are  used,  it  is  of  equal  im- 
portance to  combine  a  condenser  with  the  primary.  One  may 
use  a  condenser  connected  to  the  ends  of  the  primary  or  to  the 
terminals  of  the  alternator,  but  the  latter  is  not  to  be  recom- 
mended, as  the  machine  might  be  injured.  The  best  way  is 
undoubtedly  to  use  the  condenser  in  series  with  the  primary  and 
with  the  alternator,  and  to  adjust  its  capacity  so  as  to  annul  the 


326 


INVENTIONS  OF  NIKOLA  TE8LA. 


self-induction  of  both  the  latter.  The  condenser  should  be  ad- 
justable by  very  small  steps,  and  for  a  finer  adjustment  a  small 
oil  condenser  with  movable  plates  may  be  used  conveniently. 

I  think  it  best  at  this  juncture  to  bring  before  you  a  phe- 
nomenon, observed  by  me  some  time  ago,  which  to  the  purely 
scientific  investigator  may  perhaps  appear  more  interesting  than 
any  of  the  results  which  I  have  the  privilege  to  present  to  you 
this  evening. 

It  may  be  quite  properly  ranked  among  the  brush  phenom- 
ena— in  fact,  it  is  a  brush,  formed  at,  or  near,  a  single  terminal 
in  high  vacuum. 

In  bulbs  provided  with  a  conducting  terminal,  though  it  be  of 


FIG.  141. 


FIG.  142. 


aluminum,  the  brush  -has  but  an  ephemeral  existence,  and  can- 
not, unfortunately,  be  indefinitely  preserved  in  its  most  sensi- 
tive state,  even  in  a  bulb  devoid  of  any  conducting  electrode. 
In  studying  the  phenomenon,  by  all  means  a  bulb  having  no 
leading-in  wire  should  be  used.  I  have  found  it  best  to  use 
bulbs  constructed  as  indicated  in  Figs.  141  and  142. 

In  Fig.  141  the  bulb  comprises  an  incandescent  lamp  globe  Z, 
in  the  neck  of  which  is  sealed  a  barometer  tube  &,  the  end  of  which 
is  blown  out  to  form  a  small  sphere  s.  This  sphere  should  be 
sealed  as  closely  as  possible  in  the  centre  of  the  large  globe. 
Before  sealing,  a  thin  tube  t,  of  aluminum  sheet,  may  be  slipped 
in  the  barometer  tube,  but  it  is  not  important  to  employ  it. 


HIGH  FREQUENCY  AND  HIGH  POTENTIAL  CURRENTS.      227 

The  small  hollow  sphere  s  is  filled  with  some  conducting 
powder,  and  a  wire  w  is  cemented  in  the  neck  for  the  purpose  of 
connecting  the  conducting  powder  with  the  generator. 

The  construction  shown  in  Fig.  142  was  chosen  in  order  to 
remove  from  the  brush  any  conducting  body  which  might  possi- 
bly affect  it.  The  bulb  consists  in  this  case  of  a  lamp  globe  Z, 
which  has  a  neck  n,  provided  with  a  tube  b  and  small  sphere  s, 
sealed  to  it,  so  that  two  entirely  independent  compartments  are 
formed,  as  indicated  in  the  drawing.  When  the  bulb  is  in  use 
the  neck  n  is  provided  with  a  tinfoil  coating,  which  is  connected 
to  the  generator  and  acts  inductively  upon  the  moderately  rare- 
fied and  highly  conducted  gas  inclosed  in  the  neck.  From  there 
the  current  passes  through  the  tube  b  into  the  small  sphere  *,  to 
act  by  induction  upon  the  gas  contained  in  the  globe  L. 

It  is  of   advantage  to  make  the   tube  £-very  thick,  the  hole 


FIG.  143. 


through  it  very  small,  and  to  blow  the  sphere  *  very  thin.  It  is 
of  the  greatest  importance  that  the  sphere  *  be  placed  in  the 
centre  of  the  globe  L. 

Figs.  143,  144  and  145  indicate  different  forms,  or  stages,  of 
the  brush.  Fig.  143  shows  the  brush  as  it  first  appears  in  a  bulb 
provided  with  a  conducting  terminal ;  but,  as  in  such  a  bulb  it 
very  soon  disappears — often  after  a  few  minutes — I  will  confine 
myself  to  the  description  of  the  phenomenon  as  seen  in  a  bulb 
without  conducting  electrode.  It  is  observed  under  the  follow- 
ing conditions : 

When  the  globe  L  (Figs.  141  and  142)  is  exhausted  to  a  very 
high  degree,  generally  the  bulb  is  not  excited  upon  connecting 
the  wire  w  (Fig.  141)  or  the  tinfoil  coating  of  the  bulb  (Fig. 


INVENTIONS  OF  NIKOLA  TESLA. 


142)  to  the  terminal  of  the  induction  coil.  To  excite  it,  it  is 
usually  sufficient  to  grasp  the  globe  L  with  the  hand.  An  in- 
tense phosphorescence  then  spreads  at  tirst  over  the  globe,  but 
soon  gives  place  to  a  white,  misty  light.  Shortly  afterward  one 
may  notice  that  the  luminosity  is  unevenly  distributed  in  the 
globe,  and  after  passing  the  current  for  some  time  the  bulb  ap- 
pears as  in  Fig.  144.  From  this  stage  the  phenomenon  will 
gradually  pass  to  that  indicated  in  Fig.  145,  after  some  minutes, 
hours,  days  or  weeks,  according  as  the  bulb  is  worked.  Warm- 
ing the  bulb  or  increasing  the  potential  hastens  the  transit. 

When  the  brush  assumes  the  form  indicated  in  Fig.  145,  it  may 
be  brought  to  a  state  of   extreme  sensitiveness  to  electrostatic 


FIG.  144. 


FIG.  145. 


and  magnetic  influence.  The  bulb  hanging  straight  down  from 
a  wire,  and  all  objects  being  remote  from  it,  the  approach  of  the 
observer  at  a  few  paces  from  the  bulb  will  cause  the  brush  to  fly 
to  the  opposite  side,  and  if  he  walks  around  the  bulb  it  will 
always  keep  on  the  opposite  side.  It  may  begin  to  spin  around 
the  terminal  long  before  it  reaches  that  sensitive  stage.  When 
it  begins  to  turn  around,  principally,  but  also  before,  it  is  affected 
by  a  magnet,  and  at  a  certain  stage  it  is  susceptible  to  magnetic 
influence  to  an  astonishing  degree.  A  small  permanent  magnet, 
with  its  poles  at  a  distance  of  no  more  than  two  centimetres,  will 
aft'ect  it  visibly  at  a  distance  of  two  metres,  slowing  down  or  ac- 
•elerating  the  rotation  according  to  how  it  is  held  relatively  to 


HIGH  FREQUENCY  AND  HIGH  POTENTIAL  CURRENTS.     229 

the  brush.  I  think  I  have  observed  that  at  the  stage  when  it  is 
most  sensitive  to  magnetic,  it  is  not  most  sensitive  to  electrostatic, 
influence.  My  explanation  is,  that  the  electrostatic  attraction 
between  the  brush  and  the  glass  of  the  bulb,  which  retards  the 
rotation,  grows  much  quicker  than  the  magnetic  influence  when 
the  intensity  of  the  stream  is  increased. 

When  the  bulb  hangs  with  the  globe  L  down,  the  rotation  is 
always  clockwise.  In  the  southern  hemisphere  it  would  occur 
in  the  opposite  direction  and  on  the  equator  the  brush  should 
not  turn  at  all.  The  rotation  may  be  reversed  by  a  magnet  kept 
at  some  distance.  The  brush  rotates  best,  seemingly,  when  it  is 
at  right  angles  to  the  lines  of  force  of  the  earth.  It  very  likely 
rotates,  when  at  its  maximum  speed,  in  synchronism  with  the 
alternations,  say,  10,000  times  a  second,  The  rotation  can  be 
slowed  down  or  accelerated  by  the  approach  or  receding  of  the 
observer,  or  any  conducting  body,  but  it  cannot  be  reversed  by 
putting  the  bulb  in  any  position.  When  it  is  in  the  state  of  the 
highest  sensitiveness  and  the  potential  or  frequency  be  varied, 
the  sensitiveness  is  rapidly  diminished.  Changing  either  of 
these  but  little  will  generally  stop  the  rotation.  The  sensitive- 
ness is  likewise  affected  by  the  variations  of  temperature.  To 
attain  great  sensitiveness  it  is  necessary  to  have  the  small  sphere 
s  in  the  centre  of  the  globe  Z,  as  otherwise  the  electrostatic 
action  of  the  glass  of  the  globe  will  tend  to  stop  the  rotation. 
The  sphere  s  should  be  small  and  of  uniform  thickness  ;  any  dis- 
symmetry of  course  has  the  effect  to  diminish  the  sensitiveness. 

The  fact  that  the  brush  rotates  in  a  delinite  direction  in  a  per- 
manent magnetic  tield  seems  to  show  that  in  alternating  currents 
of  very  high  frequency  the  positive  and  negative  impulses  are 
not  equal,  but  that  one  always  preponderates  over  the  other. 

Of  course,  this  rotation  in  one  direction  may  be  due  to  the 
action  of  the  two  elements  of  the  same  current  upon  each  other, 
or  to  the  action  of  the  field  produced  by  one  of  the  elements 
upon  the  other,  as  in  a  series  motor,  without  necessarily  one  im- 
pulse being  stronger  than  the  other.  The  fact  that  the  brush 
turns,  as  far  as  I  could  observe,  in  any  position,  would  speak  for 
this  view.  In  such  case  it  would  turn  at  any  point  of  the  earth's 
surface.  But,  on  the  other  hand,  it  is  then  hard  to  explain  why 
a  permanent  magnet  should  reverse  the  rotation,  and  one  must 
assume  the  preponderance  of  impulses  of  one  kind. 

As  to  the  causes  of   the  formation  of  the  brush  or  stream,  i 


230  INVENTIONS  OF  NIKOLA  TESLA. 

think  it  is  due  to  the  electrostatic  action  of  the  globe  and  the 
dissymmetry  of  the  parts.  If  the  small  bulb  *  and  the  globe  Z 
were  perfect  concentric  spheres,  and  the  glass  throughout  of  the 
same  thickness  and  quality,  I  think  the  brush  would  not  form, 
as  the  tendency  to  pass  would  be  equal  on  all  sides.  That  the 
formation  of  the  stream  is  due  to  an  irregularity  is  apparent  from 
the  fact  that  it  has  the  tendency  to  remain  in  one  position,  and 
rotation  occurs  most  generally  only  when  it  is  brought  out  of 
this  position  by  electrostatic  or  magnetic  influence.  When  in  an 
extremely  sensitive  state  it  rests  in  one  position,  most  curious  ex- 
periments may  be  performed  with  it.  For  instance,  the  experi- 
menter may,  by  selecting  a  proper  position,  approach  the  hand 
at  a  certain  considerable  distance  to  the  bulb,  and  he  may  cause 
the  brush  to  pass  oif  by  merely  stiffening  the  muscles  of  the  arm. 
When  it  begins  to  rotate  slowly,  and  the  hands  are  held  at  a 
proper  distance,  it  is  impossible  to  make  even  the  slightest  motion 
without  producing  a  visible  effect  upon  the  brush.  A  metal 
plate  connected  to  the  other  terminal  of  the  coil  affects  it  at  a 
great  distance,  slowing  down  the  rotation  often  to  one  turn  a 
second. 

I  am  firmly  convinced  that  such  a  brush,  when  we  learn  how 
to  produce  it  properly,  will  prove  a  valuable  aid  in  the  investi- 
gation of  the  nature  of  the  forces  acting  in  an  electrostatic  or 
magnetic  field.  If  there  is  any  motion  which  is  measurable  going 
on  in  the  space,  such  a  brush  ought  to  reveal  it.  It  is,  so  to 
speak,  a  beam  of  light,  frictionless,  devoid  of  inertia. 

I  think  that  it  may  find  practical  applications  in  telegraphy. 
With  such  a  brush  it  would  be  possible  to  send  dispatches  across 
the  Atlantic,  for  instance,  with  any  speed,  since  its  sensitiveness 
may  be  so  great  that  the  slightest  changes  will  affect  it.  If  it 
were  possible  to  make  the  stream  more  intense  and  very  narrow, 
its  deflections  could  be  easily  photographed. 

I  have  been  interested  to  find  whether  there  is  a  rotation  of 
the  stream  itself,  or  whether  there  is  simply  a  stress  traveling 
around  the  bulb.  For  this  purpose  I  mounted  a  light  mica  fan 
so  that  its  vanes  were  in  the  path  of  the  brush.  If  the  stream 
itself  was  rotating  the  fan  would  be  spun  around.  I  could  pro- 
duce no  distinct  rotation  of  the  fan,  although  I  tried  the  experi- 
ment repeatedly ;  but  as  the  fan  exerted  a  noticeable  influence 
on  the  stream,  and  the  apparent  rotation  of  the  latter  was,  in  this 
case,  never  quite  satisfactory,  the  experiment  did  not  appear  to 
be  conclusive. 


HIGH  FREQUENCY  AND  HIGH  POTENTIAL  CURRENTS.     231 

I  have  been  unable  to  produce  the  phenomenon  with  the  dis- 
ruptive discharge  coil,  although  every  other  of  these  phenomena 
can  be  well  produced  by  it — many,  in  fact,  much  better  than 
with  coils  operated  from  an  alternator. 

It  may  be  possible  to  produce  the  brush  by  impulses  of  one 
direction,  or  even  by  a  steady  potential,  in  which  case  it  would 
be  still  more  sensitive  to  magnetic  influence. 

In  operating  an  induction  coil  with  rapidly  alternating  currents, 
we  realize  with  astonishment,  for  the  first  time,  the  great  import- 
ance of  the  relation  of  capacity,  self-induction  and  frequency  as 
regards  the  general  results.  The  effects  of  capacity  are  the  most 
striking,  for  in  these  experiments,  since  the  self-induction  and 
frequency  both  are  high,  the  critical  capacity  is  very  small,  and 
need  be  but  slightly  varied  to  produce  a  very  considerable  change- 
The  experimenter  may  bring  his  body  in  contact  with  the  ter- 
minals of  the  secondary  of  the  coil,  or  attach  to  one  or  both  ter- 
minals insulated  bodies  of  very  small  bulk,  such  as  bulbs,  and  lie 
may  produce  a  considerable  rise  or  fall  of  potential,  and  greatly 
affect  the  now  of  the  current  through  the  primary.  In  the  ex- 
periment before  shown,  in  which  a  brush  appears  at  a  wire 
attached  to  one  terminal,  and  the  wire  is  vibrated  when  the  ex- 
perimenter brings  his  insulated  body  in  contact  with  the  other 
terminal  of  the  coil,  the  sudden  rise  of  potential  was  made  evi- 
dent, 

I  may  show  you  the  behavior  of  the  coil  in  another  manner 
which  possesses  a  feature  of  some  interest.  I  have  here  a  little  light 
fan  of  aluminum  sheet,  fastened  to  a  needle  and  arranged  to 
rotate  freely  in  a  metal  piece  screwed  to  one  of  the  terminals  of 
the  coil.  When  the  coil  is  set  to  work,  the  molecules  of  the  air 
are  rhythmically  attracted  and  repelled.  As  the  force  with 
which  they  are  repelled  is  greater  than  that  with  which  they  are 
attracted,  it  results  that  there  is  a  repulsion  exerted  on  the  sur- 
faces of  the  fan.  If  the  fan  were  made  simply  of  a  metal  sheet, 
the  repulsion  would  be  equal  on  the  opposite  sides,  and  would 
produce  no  effect.  But  if  one  of  the  opposing  surfaces  is  screen- 
ed, or  if,  generally  speaking,  the  bombardment  on  this  side  is 
weakened  in  some  way  or  other,  there  remains  the  repulsion  ex- 
erted upon  the  other,  and  the  fan  is  set  in  rotation.  The  screen- 
ing is  best  effected  by  fastening  upon  one  of  the  opposing  sides 
of  the  fan  insulated  conducting  coatings,  or,  if  the  fan  is  made 
in  the  shape  of  an  ordinary  propeller  screw,  by  fastening  on  one 


•232  INVENTIONS  OF  NIKOLA  TESLA. 

side,  and  close  to  it,  an  insulated  metal  plate.  The  static  screen 
may,  however,  be  omitted,  and  simply  a  thickness  of  insulating 
material  fastened  to  one  of  the  sides  of  the  fan. 

To  show  the  behavior  of  the  coil,  the  fan  may  be  placed  upon 
the  terminal  and  it  will  readily  rotate  when  the  coil  is  operated 
by  currents  of  very  high  frequency.  With  a  steady  potential, 
of  course,  and  even  with  alternating  currents  of  very  low  fre- 
quency, it  would  not  turn,  because  of  the  very  slow  exchange  of 
air  and,  consequently,  smaller  bombardment;  but  in  the  latter 
case  it  might  turn  if  the  potential  were  excessive.  With  a  pin 
wheel,  quite  the  opposite  rule  holds  good;  it  rotates  best  with 
a  steady  potential,  and  the  eifort  is  the  smaller  the  higher  the 
frequency.  Now,  it  is  very  easy  to  adjust  the  conditions  so  that 
the  potential  is  normally  not  sufficient  to  turn  the  fan,  but  that 
by  connecting  the  other  terminal  of  the  coil  with  an  insulated 
body  it  rises  to  a  much  greater  value,  so  as  to  rotate  the  fan,  and 
it  is  likewise  possible  to  stop  the  rotation  by  connecting  to  the 
terminal  a  body  of  different  size,  thereby  diminishing  the  potent- 
ial. 

Instead  of  using  the  fan  in  this  experiment,  we  may  use  the 
"  electric  "  radiometer  with  similar  effect.  But  in  this  case  it  will 
be  found  that  the  vanes  will  rotate  only  at  high  exhaustion  or  at 
ordinary  pressures;  they  will  not  rotate  at  moderate  pressures, 
when  the  air  is  highly  conducting.  This  curious  observation  was 
made  conjointly  by  Professor  Crcokes  and  myself.  I  attribute 
the  result  to  the  high  conductivity  of  the  air,  the  molecules  of 
which  then  do  not  act  as  independent  carriers  of  electric  charges, 
but  act  all  together  as  a  single  conducting  body.  In  such  case, 
of  course,  if  there  is  any  repulsion  at  all  of  the  molecules  from 
the  vanes,  it  must  be  very  small.  It  is  possible,  however,  that 
the  result  is  in  part  due  to  the  fact  that  the  greater  part  of  the 
discharge  passes  from  the  leading-in  wire  through  the  highly  con- 
ducting gas,  instead  of  passing  off  from  the  conducting  vanes. 

In  trying  the  preceding  experiment  with  the  electric  radiometer 
the  potential  should  not  exceed  a  certain  limit,  as  then  the  elec- 
trostatic attraction  between  the  vanes  and  the  glass  of  the  bulb 
may  be  so  great  as  to  stop  the  rotation. 

A  most  curious  feature  of  alternate  currents  of  high  frequen- 
cies and  potentials  is  that  they  enable  us  to  perform  many  experi- 
ments by  the  use  of  one  wire  only.  In  many  respects  this  feat, 
ure  is  of  great  interest. 


HIGH  FREQUENCY  AND  HIGH  POTENTIAL  CURRENTS.     233 

In  a  type  of  alternate  current  motor  invented  by  me  some  years 
ago  I  produced  rotation  by  inducing,  by  means  of  a  single  alter- 
nating current  passed  through  a  motor  circuit,  in  the  mass  or  other 
circuits  of  the  motor,  secondary  currents,  which,  jointly  with  the 
primary  or  inducing  current,  created  a  moving  field  of  force.  A 
simple  but  crude  form  of  such  a  motor  is  obtained  by  winding 
upon  an  iron  core  a  primary,  and  close  to  it  a  secondary  coil,  join- 
ing the  ends  of  the  latter  and  placing  a  freely  movable  metal  disc 
within  the  influence  of  the  field  produced  by  both.  The  iron  core 
is  employed  for  obvious  reasons,  but  it  is  not  essential  to  the 
operation.  To  improve  the  motor,  the  iron  core  is  made  to  en- 
circle the  armature.  Again  to  improve,  the  secondary  coil  is 
made  to  partly  overlap  the  primary,  so  that  it  cannot  free  itself 
from  a  strong  inductive  action  of  the  latter,  repel  its  lines  as  it 
may.  Once  more  to  improve,  the  proper  difference  of  phase  is 
obtained  between  the  primary  and  secondary  currents  by  a  con- 
denser, self-induction,  resistance  or  equivalent  windings. 

I  had  discovered,  however,  that  rotation  is  produced  by  means 
of  a  single  coil  and  core;  my  explanation  of  the  phenomenon,  and 
leading  thought  in  trying  the  experiment,  being  that  there  must 
be  a  true  time  lag  in  the  magnetization  of  the  core.  I  remember 
the  pleasure  I  had  when,  in  the  writings  of  Professor  Ayrton, 
which  came  later  to  my  hand,  I  found  the  idea  of  the  time  lag 
advocated.  Whether  there  is  a  true  time  lag,  or  whether  the  re- 
tardation is  due  to  eddy  currents  circulating  in  minute  paths,  must 
remain  an  open  question,  but  the  fact  is  that  a  coil  wound  upon 
an  iron  core  and  traversed  by  an  alternating  current  creates  a 
moving  field  of  force,  capable  of  setting  an  armature  in  rotation. 
It  is  of  some  interest,  in  conjunction  with  the  historical  Arago 
experiment,  to  mention  that  in  lag  or  phase  motors  I  have  pro- 
duced rotation  in  the  opposite  direction  to  the  moving  field,  which 
means  that  in  that  experiment  the  magnet  may  not  rotate,  or  may 
even  rotate  in  the  opposite  direction  to  the  moving  disc.  Here, 
then,  is  a  motor  (diagrammatically  illustrated  in  Fig.  146),  com- 
prising a  coil  and  iron  core,  and  a  freely  movable  copper  disc  in 
proximity  to  the  latter. 

To  demonstrate  a  novel  and  interesting  feature,  I  have,  for  a 
reason  which  I  will  explain,  selected  this  type  of  motor.  When 
the  ends  of  the  coil  are  connected  to  the  terminals  of  an  alter- 
nator the  disc  is  set  in  rotation.  But  it  is  not  this  experiment, 
now  well  known,  which  I  desire  to  perform.  What  I  wish  to 


234  INVENTIONS  OF  NIKOLA  TESLA. 

show  you  is  that  this  motor  rotates  with  one  single  connection  be- 
tween it  and  the  generator;  that  is  to  say,  one  terminal  of  the 
motor  is  connected  to  one  terminal  of  the  generator — in  this  case 
the  secondary  of  a  high-tension  induction  coil — the  other  term- 
inals of  motor  and  generator  being  insulated  in  space.  To  pro- 
duce rotation  it  is  generally  ( but  not  absolutely )  necessary  to 
connect  the  free  end  of  the  motor  coil  to  an  insulated  body  of 
some  size.  The  experimenter's  body  is  more  than  sufficient.  If 
he  touches  the  free  terminal  with  an  object  held  in  the  hand,  a 
current  passes  through  the  coil  and  the  copper  disc  is  set  in  rota- 
tion. If  an  exhausted  tube  is  put  in  series  with  the  coil,  the  tube 
lights  brilliantly,  showing  the  passage  of  a  strong  current.  In- 


PIG.  146. 

stead  of  the  experimenter's  body,  a  small  metal  sheet  suspended 
on  a  cord  may  be  used  with  the  same  result.  In  this  case  the 
plate  acts  as  a  condenser  in  series  with  the  coil.  It  counteracts 
the  self-induction  of  the  latter  and  allows  a  strong  current  to 
pass.  In  such  a  combination,  the  greater  the  self-induction  of 
the  coil  the  smaller  need  be  the  plate,  and  this  means  that  a  lower 
frequency,  or  eventually  a  lower  potential,  is  required  to  operate 
the  motor.  A  single  coil  wound  upon  a  core  has  a  high  self- 
induction  ;  for  this  reason,  principally,  this  type  of  motor  was 
chosen  to  perform  the  experiment.  Were  a  secondary  closed 
coil  wound  upon  the  core,  it  would  tend  to  diminish  the  self- 


HIGH  FREQUENCY  AND  HIGH  POTENTIAL  CURRENTS.     285 

induction,  and  then  it  would  be  necessary  to  employ  a  much 
higher  frequency  and  potential.  Neither  would  be  advisable,  for 
a  higher  potential  would  endanger  the  insulation  of  the  small 
primary  coil,  and  a  higher  frequency  would  result  in  a  materially 
diminished  torque. 

It  should  be  remarked  that  when  such  a  motor  with  a 
closed  secondary  is  used,  it  is  not  at  all  easy  to  obtain  rota- 
tion with  excessive  frequencies,  as  the  secondary  cuts  off 
almost  completely  the  lines  of  the  primary — and  this,  of 
course,  the  more,  the  higher  the  frequency — and  allows  the  pass- 
age of  but  a  minute  current.  In  such  a  case,  unless  the  second- 
ary is  closed  through  a  condenser,  it  is  almost  essential,  in  order 
to  produce  rotation,  to  make  the  primary  and  secondary  coils 
overlap  each  other  more  or  less. 

But  there  is  an  additional  feature  of  interest  about  this  motor, 
namely,  it  is  not  necessary  to  have  even  a  single  connection  be- 
tween the  motor  and  generator,  except,  perhaps,  through  the 
ground;  for  not  only  is  an  insulated  plate  capable  of  giving  off 
energy  into  space,  but  it  is  likewise  capable  of  deriving  it  from 
an  alternating  electrostatic  field,  though  in  the  latter  case  the 
available  energy  is  much  smaller.  In  this  instance  one  of  the 
motor  terminals  is  connected  to  the  insulated  plate  or  body 
located  within  the  alternating  electrostatic  field,  and  the  other 
terminal  preferably  to  the  ground. 

It  is  quite  possible,  however,  that  such  "  no  wire  "  motors,  as 
they  might  be  called,  could  be  operated  by  conduction  through 
the  rarefied  air  at  considerable  distances.  Alternate  currents, 
especially  of  high  frequencies,  pass  with  astonishing  freedom 
through  even  slightly  rarefied  gases.  The  upper  strata  of  the  air 
are  rarefied.  To  reach  a  number  of  miles  out  into  space  requires 
the  overcoming  of  difficulties  of  a  merely  mechanical  nature. 
There  is  no  doubt  that  with  the  enormous  potentials  obtainable  by 
the  use  of  high  frequencies  and  oil  insulation,  luminous  discharges 
might  be  passed  through  many  miles  of  rarefied  air,  and  that,  by 
thus  directing  the  energy  of  many  hundreds  or  thousands  of  horse- 
power, motors  or  lamps  might  be  operated  at  considerable 
distances  from  stationary  sources.  But  such  schemes  are  men- 
tioned merely  as  possibilities.  We  shall  have  no  need  to  transmit 
power  in  this  way.  We  shall  have  no  need  to  transmit  power 
at  all.  Ere  many  generations  pass,  our  machinery  will  be  driven 
by  a  power  obtainable  at  any  point  of  the  universe.  This  idea  is 


236  INVENTIONS  OF  NIKOLA  TESLA. 

not  novel.  Men  have  been  led  to  it  long  ago  by  instinct  or  reason. 
It  has  been  expressed  in  many  ways,  and  in  many  places,  in  the 
history  of  old  and  new.  We  find  it  in  the  delightful  myth  of 
Antheus,  who  derives  power  from  the  earth ;  we  find  it  among 
the  subtle  speculations  of  one  of  your  splendid  mathematicians, 
and  in  many  hints  and  statements  of  thinkers  of  the  present  time. 
Throughout  space  there  is  energy.  Is  this  energy  static  or  kinetic  ? 
If  static  our  hopes  are  in  vain;  if  kinetic — and  this  we  know  it 
is,  for  certain — then  it  is  a  mere  question  of  time  when  men  will 
succeed  in  attaching  their  machinery  to  the  very  wheelwork  of 
nature.  Of  all,  living  or  dead,  Crookes  came  nearest  to  doing  it. 
His  radiometer  will  turn  in  the  light  of  day  and  in  the  darkness 
of  the  night;  it  will  turn  everywhere  where  there  is  heat,  and 
heat  is  everywhere.  But,  unfortunately,  this  beautiful  little 
machine,  while  it  goes  down  to  posterity  as  the  most  interesting, 
must  likewise  be  put  on  record  as  the  most  inefficient  machine 
ever  invented ! 

The  preceding  experiment  is  only  one  of  many  equally  inter- 
esting experiments  which  may  be  performed  by  the  use  of  only 
one  wire  with  alternations  of  high  potential  and  frequency.  We 
may  connect  an  insulated  line  to  a  source  of  such  currents,  we 
may  pass  an  inappreciable  current  over  the  line,  and  on  any 
point  of  the  same  we  are  able  to  obtain  a  heavy  current,  capable 
of  fusing  a  thick  copper  wire.  Or  we  may,  by  the  help  of  some 
artifice,  decompose  a  solution  in  any  electrolytic  cell  by  con- 
necting only  one  pole  of  the  cell  to  the  line  or  source  of  energy. 
Or  we  may,  by  attaching  to  the  line,  or  only  bringing  into  its 
vicinity,  light  up  an  incandescent  lamp,  an  exhausted  tube,  or  a 
phosphorescent  bulb. 

However  impracticable  this  plan  of  working  may  appear  in 
many  cases,  it  certainly  seems  practicable,  and  even  recommend- 
able,  in  the  production  of  light.  A  perfected  lamp  would  require 
but  little  energy,  and  if  wires  were  used  at  all  we  ought  to  be  able 
to  supply  that  energy  without  a  return  wire. 

It  is  now  a  fact  that  a  body  may  be  rendered  incandescent  or 
phosphorescent  by  bringing  it  either  in  single  contact  or  merely 
in  the  vicinity  of  a  source  of  electric  impulses  of  the  proper 
character,  and  that  in  this  manner  a  quantity  of  light  sufficient 
to  afford  a  practical  illuminant  may  be  produced.  It  is,  there- 
fore, to  say  the  least,  worth  while  to  attempt  to  determine  the 
best  conditions  and  to  invent  the  best  appliances  for  attaining 
this  object. 


HIGH  FREQUENCY  AND  HIGH  POTENTIAL  CURRENTS.      237 

Some  experiences  have  already  been  gained  in  this  direction, 
and  I  will  dwell  on  them  briefly,  in  the  hope  that  they  might 
prove  useful. 

The  heating  of  a  conducting  body  inclosed  in  a  bulb,  and  con- 
nected to  a  source  of  rapidly  alternating  electric  impulses,  is 
dependent  on  so  many  things  of  a  different  nature,  that  it  would 
be  difficult  to  give  a  generally  applicable  rule  under  which  the 
maximum  heating  occurs.  As  regards  the  size  of  the  vessel,  I 
have  lately  found  that  at  ordinary  or  only  slightly  differing 
atmospheric  pressures,  when  air  is  a  good  insulator,  and  hence 
practically  the  same  amount  of  energy  by  a  certain  potential  and 
frequency  is  given  off  from  the  body,  whether  the  bulb  be  small 
or  large,  the  body  is  brought  to  a  higher  temperature  if  enclosed 
in  a  small  bulb,  because  of  the  better  confinement  of  heat  in  this 
case. 

At  lower  pressures,  when  air  becomes  more  or  less  conducting, 
or  if  the  air  be  sufficiently  warmed  to  become  conducting,  the 
body  is  rendered  more  intensely  incandescent  in  a  large  bulb, 
obviously  because,  under  otherwise  equal  conditions  of  test,  more 
energy  may  be  given  off  from  the  body  when  the  bulb  is  large. 

At  very  high  degrees  of  exhaustion,  when  the  matter  in  the 
bulb  becomes  "  radiant,"  a  large  bulb  has  still  an  advantage,  but 
a  comparatively  slight  one,  over  the  small  bulb. 

Finally,  at  excessively  high  degrees  of  exhaustion,  which  can- 
not be  reached  except  by  the  employment  of  special  means,  there 
seems  to  be,  beyond  a  certain  and  rather  small  size  of  vessel,  no 
perceptible  difference  in  the  heating-. 

These  observations  were  the  result  of  a  number  of  experiments, 
of  which  one,  showing  the  effect  of  the  size  of  the  bulb  at  a  high 
degree  of  exhaustion,  may  be  described  and  shown  here,  as  it 
presents  a  feature  of  interest.  Three  spherical  bulbs  of  2  inches, 
3  inches  and  4  inches  diameter  were  taken,  and  in  the  centre  of 
each  was  mounted  an  equal  length  of  an  ordinary  incandescent 
lamp  filament  of  uniform  thickness.  In  each  bulb  the  piece  of 
filament  was  fastened  to  the  leading-in  wire  of  platinum,  con- 
tained in  a  glass  stem  sealed  in  the  bulb ;  care  being  taken,  of 
course,  to  make  everything  as  nearly  alike  as  possible.  On  each 
glass  stem  in  the  inside  of  the  bulb  was  slipped  a  highly  polished 
tube  made  of  aluminum  sheet,  which  fitted  the'stem  and  was  held 
on  it  by  spring  pressure.  The  function  of  this  aluminum  tube  will 
bo  explained  subsequently.  In  each  bulb  an  equal  length  of  fila- 


238  INVENTIONS  OF  NIKOLA  TESLA. 

ment  protruded  above  the  metal  tube.  It  is  sufficient  to  say  now 
that  under  these  conditions  equal  lengths  of  filament  of  the  same 
thickness — in  other  words,  bodies  of  equal  bulk — were  brought 
to  incandescence.  The  three  bulbs  were  sealed  to  a  glass  tube, 
which  was  connected  to  a  Sprengel  pump.  When  a  high  vacuum 
had  been  reached,  the  glass  tube  carrying  the  bulbs  was  sealed 
off.  A  current  was  then  turned  on  successively  on  each  bulb, 
and  it  was  found  that  the  filaments  came  to  about  the  same 
brightness,  and,  if  anything,  the  smallest  bulb,  which  was  placed 
midway  between  the  two  larger  ones,  may  have  been  slightly 
brighter.  This  result  was  expected,  for  when  either  of  the  bulbs 
was  connected  to  the  coil  the  luminosity  spread  through  the 
other  two,  hence  the  three  bulbs  constituted  really  one  vessel. 
When  all  the  three  bulbs  were  connected  in  multiple  arc  to  the 
coil,  in  the  largest  of  them  the  filament  glowed  brightest,  in  the 
next  smaller  it  was  a  little  less  bright,  and  in  the  smallest  it  only 
came  to  redness.  The  bulbs  were  then  sealed  off  and  separately 
tried.  The  brightness  of  the  filaments  was  now  such  as  would 
have  been  expected  on  the  supposition  that  the  energy  given  off 
was  proportionate  to  the  surface  of  the  bulb,  this  surface  in  each 
case  representing  one  of  the  cpatings  of  a  condenser.  Accord- 
ingly, there  was  less  difference  between  the  largest  and  the 
middle  sized  than  between  the  latter  and  the  smallest  bulb. 

An  interesting  observation  was  made  in  this  experiment.  The 
three  bulbs  were  suspended  from  a  straight  bare  wire  connected 
to  a  terminal  of  a  coil,  the  largest  bulb  being  placed  at  the  end 
of  the  wire,  at  some  distance  from  it  the  smallest  bulb,  and  at  an 
equal  distance  from  the  latter  the  middle-sized  one.  The  carbons 
glowed  then  in  both  the  larger  bulbs  about  as  expected,  but  the 
smallest  did  not  get  its  share  by  far.  This  observation  led  me  to 
exchange  the  position  of  the  bulbs,  and  I  then  observed  that 
whichever  of  the  bulbs  was  in  the  middle  was  by  far  less  bright 
than  it  was  in  any  other  position.  This  mystifying  result  was, 
of  course,  found  to  be  due  to  the  electrostatic  action  between  the 
bulbs.  When  they  were  placed  at  a  considerable  distance,  or 
when  they  were  attached  to  the  corners  of  an  equilateral  triangle 
of  copper  wire,  they  glowed  in  about  the  order  determined  by 
their  surfaces. 

As  to  the  shape  of  the  vessel,  it  is  also  of  some  importance,  especi- 
ally at  high  degrees  of  exhaustion.  Of  all  the  possible  construc- 
tions, it  seems  that  a  spherical  globe  with  the  refractory  body 


HIGH  FREQUENCY  AND  HIGH  POTENTIAL  CURRENTS.      239 

mounted  in  its  centre  is  the  best  to  employ.  By  experience  it 
lias  been  demonstrated  that  in  such  a  globe  a  refractory  body  of 
a  given  bulk  is  more  easily  brought  to  incandescence  than  when 
differently  shaped  bulbs  are  used.  There  is  also  an  advantage  in 
giving  to  the  incandescent  body  the  shape  of  a  sphere,  for  self- 
evident  reasons.  In  any  case  the  body  should  be  mounted  in  the 
centre,  where  the  atoms  rebounding  from  the  glass  collide.  This 
object  is  best  attained  in  the  spherical  bulb ;  but  it  is  also  at- 
tained in  a  cylindrical  vessel  with  one  or  two  straight  filaments 
coinciding  with  its  axis,  and  possibly  also  in  parabolical  or  spheri- 
cal bulbs  with  refractory  body  or  bodies  placed  in  the  focus  or 
foci  of  the  same;  though  the  latter  is  not  probable,  as  the  elec- 
trified atoms  should  in  all  cases  rebound  normally  from  the 
surface  they  strike,  unless  the  speed  were  excessive,  in  which 
case  they  would  probably  follow  the  general  law  of  reflection. 
]Sro  matter  what  shape  the  vessel  may  have,  if  the  exhaustion  be 
low,  a  filament  mounted  in  the  globe  is  brought  to  the  same 
degree  of  incandescence  in  all  parts ;  but  if  the  exhaustion  be 
high  and  the  bulb  be  spherical  or  pear-shaped,  as  usual,  focal 
points  form  and  the  filament  is  heated  to  a  higher  degree  at  or 
near  such  points. 

To  illustrate  the  effect,  I  have  here  two  small  bulbs  which  are 
alike,  only  one  is  exhausted  to  a  low  and  the  other  to  a  very  high 
degree.  When  connected  to  the  coil,  the  filament  in  the  former 
glows  uniformly  throughout  all  its  length  ;  whereas  in  the  latter, 
that  portion  of  the  filament  which  is  in  the  centre  of  the  bulb 
glows  far  more  intensely  than  the  rest.  A  curious  point  is  that 
the  phenomenon  occurs  even  if  two  filaments  are  mounted  in  a 
bulb,  each  being  connected  to  one  terminal  of  the  coil,  and,  what 
is  still  more  curious,  if  they  be  very  near  together,  provided  the 
vacuum  be  very  high.  I  noted  in  experiments  with  such  bulbs 
that  the  filaments  would  give  way  usually  at  a  certain  point,  and 
in  the  first  trials  I  attributed  it  to  a  defect  in  the  carbon.  But 
when  the  phenomenon  occurred  many  times  in  succession  I 
recognized  its  real  cause. 

In  order  to  bring  a  refractory  body  inclosed  in  a  bulb  to  in- 
candescence, it  is  desirable,  on  account  of  economy,  that  all  the 
energy  supplied  to  the  bulb  from  the  source  should  reach  without 
loss  the  body  to  be  heated ;  from  there,  and  from  nowhere  else, 
it  should  be  radiated.  It  is,  of  course,  out  of  the  question  to 
reach  this  theoretical  result,  but  it  is  possible  by  a  proper  construc- 
tion of  the  illuminating  device  to  approximate  it  more  or  less. 


240  INVENTIONS  OF  NIKOLA  TESLA. 

For  many  reasons,  the  refractory  body  is  placed  in  the  centre 
of  the  bulb,  and  it  is  usually  supported  on  a  glass  stem  containing 
the  leading-in  wire.  As  the  potential  of  this  wire  is  alternated, 
the  rarefied  gas  surrounding  the  stem  is  acted  upon  inductively, 
and  the  glass  stem  is  violently  bombarded  and  heated.  In  this 
manner  by  far  the  greater  portion  of  the  energy  supplied  to  the 
bulb — especially  when  exceedingly  high  frequencies  are  used — 
may  be  lost  for  the  purpose  contemplated.  To  obviate  this  loss, 
or  at  least  to  reduce  it  to  a  minimum,  I  usually  screen  the  rarefied 
gas  surrounding  the  stem  from  the  inductive  action  of  the  leading-in 
wire  by  providing  the  stem  with  a  tube  or  coating  of  conducting 
material.  It  seems  beyond  doubt  that  the  best  among  metals  to 
employ  for  this  purpose  is  aluminum,  on  account  of  its  many  re- 
markable properties.  Its  only  fault  is  that  it  is  easily  fusible, 
and,  therefore,  its  distance  from  the  incandescing  body  should  be 
properly  estimated.  Usually,  a  thin  tube,  of  a  diameter  some- 
what smaller  than  that  of  the  glass  stem,  is  made  of  the  finest 
aluminum  sheet,  and  slipped  on  the  stem.  The  tube  is  conveni- 
ently prepared  by  wrapping  around  a  rod  fastened  in  a  lathe  a 
piece  of  aluminum  sheet  of  proper  size,  grasping  the  sheet  firmly 
with  clean  chamois  leather  or  blotting  paper,  and  spinning  the 
rod  very  fast.  The  sheet  is  wound  tightly  around  the  rod,  and  a 
highly  polished  tube  of  one  or  three  layers  of  the  sheet  is  obtained. 
When  slipped  on  the  stem,  the  pressure  is  generally  sufficient  to 
prevent  it  from  slipping  off,  but,  for  safety,  the  lower  edge  of 
the  sheet  may  be  turned  inside.  The  upper  inside  corner  of  the 
sheet — that  is,  the  one  which  is  nearest  to  the  refractory  incan- 
descent body — should  be  cut  out  diagonally,  as  it  often  happens 
that,  in  consequence  of  the  intense  heat,  this  corner  turns  toward 
the  inside  and  comes  very  near  to,  or  in  contact  with,  the  wire,  or 
filament,  supporting  the  refractory  body.  The  greater  part  of 
the  energy  supplied  to  the  bulb  is  then  used  up  in  heating  the 
metal  tube,  and  the  bulb  is  rendered  useless  for  the  purpose. 
The  aluminum  sheet  should  project  above  the  glass  stem  more  or 
less — one  inch  or  so — or  else,  if  the  glass  be  too  close  to  the  in- 
candescing body,  it  may  be  strongly  heated  and  become  more  or 
less  conducting,  whereupon  it  may  be  ruptured,  or  may,  by  its 
conductivity,  establish  a  good  electrical  connection  between  the 
metal  tube  and  the  leading-in  wire,  in  which  case,  again,  most  of 
the  energy  will  be  lost  in  heating  the  former.  Perhaps  the  best 
way  is  to  make  the  top  of  the  glass  tube,  for  about  an  inch,  of  a 


HIGH  FREQUENCY  AND  HIGH  POTENTIAL  CURRENTS.      241 

much  smaller  diameter.  To  still  further  reduce  the  danger 
arising  from  the  heating  of  the  glass  stem,  and  also  with  the  view 
of  preventing  an  electrical  connection  between  the  metal  tube 
and  the  electrode,  I  preferably  wrap  the  stem  with  several  layers 
of  thin  mica,  which  extends  at  least  as  far  as  the  metal  tube.  In 
some  bulbs  I  have  also  used  an  outside  insulating  cover. 

The  preceding  remarks  are  only  made  to  aid  the  experimenter 
in  the  first  trials,  for  the  difficulties  which  he  encounters  he  may 
soon  find  means  to  overcome  in  his  own  way. 

To  illustrate  the  effect  of  the  screen,  and  the  advantage  of 
using  it,  I  have  here  two  bulbs  of  the  same  size,  with  their  stems, 
leading-in  wires  and  incandescent  lamp  filaments  tied  to  the  latter, 
as  nearly  alike  as  possible.  The  stem  of  one  bulb  is  provided 
with  an  aluminum  tube,  the  stem  of  the  other  has  none.  Origi- 
nally the  two  bulbs  were  joined  by  a  tube  which  was  connected 
to  a  Sprengel  pump.  When  a  high  vacuum  had  been  reached, 
first  the  connecting  tube,  and  then  the  bulbs,  were  sealed  off ; 
they  are  therefore  of  the  same  degree  of  exhaustion.  When  they 
are  separately  connected  to  the  coil  giving  a  certain  potential,  the 
carbon  filament  in  the  bulb  provided  with  the  aluminum  screen 
is  rendered  highly  incandescent,  while  the  filament  in  the  other 
bulb  may,  with  the  same  potential,  not  even  come  to  redness, 
although  in  reality  the  latter  bulb  takes  generally  more  energy 
than  the  former.  When  they  are  both  connected  together  to  the 
terminal,  the  difference  is  even  more  apparent,  showing  the  impor- 
tance of  the  screening.  The  metal  tube  placed  on  the  stem  contain- 
ing the  leading-in  wire  performs  really  two  distinct  functions:  First, 
it  acts  more  or  less  as  an  electrostatic  screen,  thus  economizing 
the  energy  supplied  to  the  bulb ;  and,  second,  to  whatever  extent 
it  may  fail  to  act  electrostatically,  it  acts  mechanically,  prevent- 
ing the  bombardment,  and  consequently  intense  heating  and 
possible  deterioration  of  the  slender  support  of  the  refractory  in- 
candescent body,  or  of  the  glass  stem  containing  the  leading-in 
wire.  I  say  slender  support,  for  it  is  evident  that  in  order  to 
confine  the  heat  more  completely  to  the  incandescing  body  its  sup- 
port should  be  very  thin,  so  as  to  carry  away  the  smallest  possible 
amount  of  heat  by  conduction.  Of  all  the  supports  used  I  have 
found  an  ordinary  incandescent  lamp  filament  to  be  the  best, 
principally  because  among  conductors  it  can  withstand  the  high- 
est degree  of  heat. 

The  effectiveness  of  the  metal  tube  as  an  electrostatic  screen 
depend?  largely  on  the  degree  of  exhaustion. 


242,  INVENTIONS  OF  NIKOLA  TESLA. 

At  excessively  high  degrees  of  exhaustion — which  are  reached 
by  using  great  care  and  special  means  in  connection  with  the 
Sprengel  pump — when  the  matter  in  the  globe  is  in  the  ultra- 
radiant  state,  it  acts  most  perfectly.  The  shadow  of  the  upper 
edge  of  the  tube  is  then  sharply  defined  upon  the  bulb. 

At  a  somewhat  lower  degree  of  exhaustion,  which  is  about  the 
ordinary  "non-striking"  vacuum,  and  generally  as  long  as  the 
matter  moves  predominantly  in  straight  Hues,  the  screen  still 
does  well.  In  elucidation  of  the  preceding  remark  it  is  necessary 
to  state  that  what  is  a  "non-striking"  vacuum  for  a  coil  operated 
as  ordinarily,  by  impulses,  or  currents,  of  low  frequency,  is  not 
so,  by  far,  when  the  coil  is  operated  by  currents  of  very  high  fre- 
quency. In  such  case  the  discharge  may  pass  with  great  freedom 
through  the  rarefied  gas  through  which  a  low  frequency  dis- 
charge may  not  pass,  even  though  the  potential  be  much  higher. 
At  ordinary  atmospheric  pressures  just  the  reverse  rule  holds 
good :  the  higher  the  frequency,  the  less  the  spark  discharge  is 
able  to  jump  between  the  terminals,  especially  if  they  are  knobs 
or  spheres  of  some  size. 

Finally,  at  very  low  degrees  of  exhaustion,  when  the  gas  is  well 
conducting,  the  metal  tube  not  only  does  not  act  as  an  electro- 
static screen,  but  even  is  a  drawback,  aiding  to  a  considerable 
extent  the  dissipation  of  the  energy  laterally  from  the  leading-in 
wire.  This,  of  course,  is  to  be  expected.  In  this  case,  namely, 
the  metal  tube  is  in  good  electrical  connection  with  the  leading- 
in  wire,  and  most  of  the  bombardment  is  directed  upon  the  tube. 
As  long  as  the  electrical  connection  is  not  good,  the  conducting 
tube  is  always  of  some  advantage,  for  although  it  may  not  greatly 
economize  energy,  still  it  protects  the  support  of  the  refractory 
button,  and  is  the  means  of  concentrating  more  energy  upon  the 
same. 

To  whatever  extent  the  aluminum  tube  performs  the  function 
of  a  screen,  its  usefulness  is  therefore  limited  to  very  high  de- 
grees of  exhaustion  when  it  is  insulated  from  the  electrode — that 
is,  when  the  gas  as  a  whole  is  non-conducting,  and  the  molecu- 
les, or  atoms,  act  as  independent  carriers  of  electric  charges. 

In  addition  to  acting  as  a  more  or  less  effective  screen,  in  the 
true  meaning  of  the  word,  the  conducting  tube  or  coating  may 
also  act,  by  reason  of  its  conductivity,  as  a  sort  of  equalizer  or 
dampener  of  the  bombardment  against  the  stem.  To  be  explicit, 
I  assume  the  action  to  be  as  follows:  Suppose  a  rhythmical  bom- 


HIGH  FREQUENCY  AND  HIGH  POTENTIAL  CURRENTS.      243 

bardment  to  occur  against  the  conducting  tube  by  reason  of  its 
imperfect  action  as  a  screen,  it  certainly  must  happen  that  some 
molecules,  or  atoms,  strike  the  tube  sooner  than  others.  Those 
which  come  first  in  contact  with  it  give  up  their  superfluous 
charge,  and  the  tube  is  electrified,  the  electrification  instantly 
spreading  over  its  surface.  But  this  must  diminish  the  energy 
lost  in  the  bombardment,  for  two  reasons :  first,  the  charge  given 
up  by  the  atoms  spreads  over  a  great  area,  and  hence  the  electric 
density  at  any  point  is  small,  and  the  atoms  are  repelled  with  less 
energy  than  they  would  be  if  they  struck  against  a  good  insu- 
lator ;  secondly,  as  the  tube  is  electrified  by  the  atoms  which  first 
come  in  contact  with  it,  the  progress  of  the  following  atoms 
against  the  tube  is  more  or  less  checked  by  the  repulsion  which 


FIG.  147.  FIG.  148. 

the  electrified  tube  must  exert  upon  the  similarly  electrified 
atoms.  This  repulsion  may  perhaps  be  sufficient  to  prevent  a 
large  portion  of  the  atoms  from  striking  the  tube,  but  at  any  rate 
it  must  diminish  the  energy  of  their  impact.  It  is  clear  that 
when  the  exhaustion  is  very  low,  and  the  rarefied  gas  well  con- 
ducting, neither  of  the  above  effects  can  occur,  and,  on  the  other 
hand,  the  fewer  the  atoms,  with  the  greater  freedom  they  move ; 
in  other  words,  the  higher  the  degree  of  exhaustion,  up  to  a 
limit,  the  more  telling  will  be  both  the  effects. 

What  I  have  just  said  may  afford  an  explanation  of  the  phe- 
nomenon observed  by  Prof.  Crookes,  namely,  that  a  discharge 
through  a  bulb  is  established  \vith  much  greater  facility  when  an 


344  INVENTIONS  OF  NIKOLA  TE8LA. 

insulator  than  when  a  conductor  is  present  in  the  same.  In  my 
opinion,  the  conductor  acts  as  a  dampener  of  the  motion  of  the 
atoms  in  the  two  ways  pointed  out ;  hence,  to  cause  a  visible  dis- 
charge to  pass  through  the  bulb,  a  much  higher  potential  is 
needed  if  a  conductor,  especially  of  much  surface,  be  present. 

For  the  sake  of  elucidating  of  some  of  the  remarks  before  made, 
I  must  now  refer  to  Figs.  147,  148  and  149,  which  illustrate 
various  arrangements  with  a  type  of  bulb  most  generally  used. 

Fig.  147  is  a  section  through  a  spherical  bulb  L,  with  the  glass 
stem  *,  contains  the  leading-in  wire  ?r,  which  has  a  lamp  filament 
I  fastened  to  it,  serving  to  support  the  refractory  button  m  in  the 
centre.  M  is  a  sheet  of  thin  mica  wound  in  several  layer*  around 
the  stem  s,  and  a  is  the  aluminum  tube. 

Fig.  148  illustrates  such  a  bulb  in  a  somewhat  more  advanced 
stage  of  perfection.  A  metallic  tube  s  is  fastened  by  means  of 
some  cement  to  the  neck  of  the  tube.  In  the  tube  is  screwed  a 
plug  P,  of  insulating  material,  in  the  centre  of  which  is  fastened 
a  metallic  terminal  t,  for  the  connection  to  the  leading-in  wire  w. 
This  terminal  must  be  well  insulated  from  the  metal  tube  s ; 
therefore,  if  the  cement  used  is  conducting — and  most  generally 
it  is  sufficiently  so — the  space  between  the  plug  P  and  the  neck 
of  the  bulb  should  be  filled  with  some  good  insulating  material, 
such  as  mica  powder. 

Fig.  149  shows  a  bulb  made  for  experimental  purposes.  In  this 
bulb  the  aluminum  tube  is  provided  with  an  external  connection, 
which  serves  to  investigate  the  eifect  of  the  tube  under  various 
conditions.  It  is  referred  to  chiefly  to  suggest  a  line  of  e.xprri- 
ment  followed. 

Since  the  bombardment  against  the  stem  containing  the  lead- 
ing-in wire  is  due  to  the  inductive  action  of  the  latter  upon  the 
rarefied  gas,  it  is  of  advantage  to  reduce  this  action  as  far  as 
practicable  by  employing  a  very  thin  wire,  surrounded  by  a  verv 
thick  insulation  of  glass  or  other  material,  and  by  making  the 
wire  passing  through  the  rarefied  gas  as  short  as  practicable.  To 
combine  these  features  I  employ  a  large  tube  T  (Fig.  150),  which 
protrudes  into  the  bulb  to  some  distance,  and  carries  on  the  top  a 
very  short  glass  stem  «,  into  which  is  sealed  the  leading-in  wire 
w,  and  I  protect  the  top  of  the  glass  stem  against  the  heat  by  a 
small  aluminum  tube  a  and  a  layer  of  mica  underneath  the  same, 
as  usual.  The  wire  u\  passing  through  the  large  tube  to  the 
outside  of  the  bulb,  should  be  well  insulated — with  a  s;lass  tube, 


HIGH  FREQUENCY  AND  HIGH  POTENTIAL  CURRENTS.      245 

for  instance — and  the  space  between  ought  to  be  filled  out  with 
some  excellent  insulator.  Among  many  insulating  powders  I 
have  found  that  mica  powder  is  the  best  to  employ.  If  this  pre- 
caution is  not  taken,  the  tube  T,  protruding  into  the  bulb,  will 
surely  be  cracked  in  consequence  of  the  heating  by  the  brushes 
which  are  apt  to  form  in  the  upper  part  of  the  tube,  near  the  ex- 
hausted globe,  especially  if  the  vacuum  be  excellent,  and  therefore 
the  potential  necessary  to  operate  the  lamp  be  very  high. 

Fig.  151  illustrates  a  similar  arrangement,  with  a  large  tube  T 
protruding  into  the  part  of  the  bulb  containing  the  refractory 
button  -ni.  In  this  case  the  wire  leading  from  the  outside  into 
the  bulb  is  omitted,  the  energy  required  being  supplied  through 


FIG.  149. 


FIG.  150. 


condenser  coatings  c  o.  The  insulating  packing  p  should  in 
this  construction  be  tightly  litting  to  the  glass,  and  rather  wide, 
or  otherwise  the  discharge  might  avoid  passing  through  the  wire 
ie,  which  connects  the  inside  condenser  coating  to  the  incandes- 
cent button  ///. 

The  molecular  bombardment  against  the  glass  stem  in  the  bulb 
is  a  source  of  great  trouble.  As  an  illustration  I  will  cite  a  phe- 
nomenon only  too  frequently  and  unwillingly  observed.  A  bulb, 
preferably  a  large  one,  may  be  taken,  and  a  good  conducting 
body,  such  as  a  piece  of  carbon,  may  be  mounted  in  it  upon  a  plati- 
num wire  sealed  in  the  glass  stem.  The  bulb  may  be  exhausted 
to  a  fairly  high  degree,  nearly  to  the  point  when  phosphorescence 


046  INVENTIONS  OF  NIKOLA  TE8LA. 

begins  to  appear.  When  the  bulb  is  connected  with  the  coil,  the 
piece  of  carbon,  if  small,  may  become  highly  incandescent  at 
first,  but  its  brightness  immediately  diminishes,  and  then  the  dis- 
charge may  break  through  the  glass  somewhere  in  the  middle  of 
the  stem,  in  the  form  of  bright  sparks,  in  spite  of  the  fact  that 
the  platinum  wire  is  in  good  electrical  connection  with  the  rare- 
fied gas  through  the  piece  of  carbon  or  metal  at  the  top.  The 
first  sparks  are  singularly  bright,  recalling  those  drawn  from  a 
clear  surface  of  mercury.  But,  as  they  heat  the  glass  rapidly, 
they,  of  course,  lose  their  brightness,  and  cease  when  the  glass  at 
the  ruptured  place  becomes  incandescent,  or  generally  sufficiently 
hot  to  conduct.  When  observed  for  the  first  time  the  phenome- 
non must  appear  very  curious,  and  shows  in  a  striking  manner 
how  radically  different  alternate  currents,  or  impulses,  of  high 
frequency  behave,  as  compared  with  steady  currents,  or  currents 
of  low  frequency.  With  such  currents — namely,  the  latter — the 
phenomenon  would  of  course  not  occur.  When  frequencies  such 
as  are  obtained  by  mechanical  means  are  used,  I  think  that  the  rup- 
ture of  the  glass  is  more  or  less  the  consequence  of  the  bombard, 
ment,  which  warms  it  up  and  impairs  its  insulating  power  ;  but 
with  frequencies  obtainable  with  condensers  I  have  no  doubt 
that  the  glass  may  give  way  without  previous  heating.  Although 
this  appears  most  singular  at  first,  it  is  in  reality  what  we  might 
expect  to  occur.  The  energy  supplied  to  the  wire  leading  into 
the  bulb  is  given  off  partly  by  direct  action  through  the  carbon 
button,  and  partly  by  inductive  action  through  the  glass  surround- 
ing the  wire.  The  case  is  thus  analogous  to  that  in  which  a  con- 
denser shunted  by  a  conductor  of  low  resistance  is  connected  to 
a  source  of  alternating  current.  As  long  as  the  frequencies  are 
low,  the  conductor  gets  the  most  and  the  condenser  is  perfectly 
safe ;  but  when  the  frequency  becomes  excessive,  the  role  of  the 
conductor  may  become  quite  insignificant.  In  the  latter  case  the 
difference  of  potential  at  the  terminals  of  the  condenser  may  be- 
come so  great  as  to  rupture  the  dielectric,  notwithstanding  the 
fact  that  the  terminals  are  joined  by  a  conductor  of  low  resis 
tance. 

It  is,  of  course,  not  necessary,  when  it  is  desired  to  produce 
the  incandescence  of  a  body  inclosed  in  a  bulb  by  means  of  these 
currents,  that  the  body  should  be  a  conductor,  for  even  a  perfect 
non-conductor  may  be  quite  as  readily  heated.  For  this  purpose 
it  is  sufficient  to  surround  a  conducting  electrode  with  a  non-con- 


HIGH  FREQUENCY  AND  HIGH  POTENTIAL  CURRENTS.     247 


material,  as,  for  instance,  in  the  bulb  described  before  in 
Fig.  150,  in  which  a  thin  incandescent  lamp  filament  is  coated 
with  a  non-conductor,  and  supports  a  button  of  the  same  material 
on  the  top.  At  the  start  the  bombardment  goes  on  by  inductive 
action  through  the  non-conductor,  until  the  same  is  sufficiently 
heated  to  become  conducting,  when  the  bombardment  continues 
in  the  ordinary  way. 

A  different  arrangement  used  in  some  of  the  bulbs  constructed 
is  illustrated  in  Fig.  152.  In  this  instance  a  non-conductor  ra  is 
mounted  in  a  piece  of  common  arc  light  carbon  so  as  to  project 
some  small  distance  above  the  latter.  The  carbon  piece  is  con- 
nected to  the  leading-ill  wire  passing  through  a  glass  stem,  which 


FIG.  151. 


FIG.  152. 


is  wrapped  with  several  layers  of  mica.  An  aluminum  tube  a  is 
employed  as  usual  for  screening.  It  is  so  arranged  that  it  reaches 
very  nearly  as  high  as  the  carbon  and  only  the  non-conductor  m 
projects  a  little  above  it.  The  bombardment  goes  at  first  against 
the  upper  surface  of  carbon,  the  lower  parts  being  protected  by 
the  aluminum  tube.  As  soon,  however,  as  the  non-conductor  m 
is  heated  it  is  rendered  good  conducting,  and  then  it  becomes  the 
centre  of  the  bombardment,  being  most  exposed  to  the  same. 

I  have  also  constructed  during  these  experiments  many  such 
single-wire  bulbs  with  or  without  internal  electrode,  in  which  the 
radiant  matter  was  projected  against,  or  focused  upon,  the  body 


248  INVENTIONS  OF  NIKOLA  TE8LA. 

to  be  rendered  incandescent.  Fig.  153  (page  263)  illustrates  one 
of  the  bulbs  used.  It  consists  of  a  spherical  globe  L,  provided 
with  a  long  neck  n,  on  top,  for  increasing  the  action  in  some  cases 
by  the  application  of  an  external  conducting  coating.  The  globe  L 
is  blown  out  on  the  bottom  into  a  very  small  bulb  Z>,  which  serves 
to  hold  it  firmly  in  a  socket  s  of  insulating  material  into  which  it 
is  cemented.  A  fine  lamp  filament  f,  supported  on  a  wire  w, 
passes  through  the  centre  of  the  globe  L.  The  filament  is  ren- 
dered incandescent  in  the  middle  portion,  where  the  bombard- 
ment proceeding  from  the  lower  inside  surface  of  the  globe  is 
most  intense.  The  lower  portion  of  the  globe,  as  far  as  the 
socket  s  reaches,  is  rendered  conducting,  either  by  a  tinfoil  coat- 
ing or  otherwise,  and  the  external  electrode  is  connected  to  a 
terminal  of  the  coil. 

The  arrangement  diagrammatically  indicated  in  Fig.  153  was 
found  to  be  an  inferior  one  when  it  was  desired  to  render  incan- 
descent a  filament  or  button  supported  in  the  centre  of  the  globe, 
but  it  was  convenient  when  the  object  was  to  excite  phosphor- 
escence. 

In  many  experiments  in  which  bodies  of  different  kind  were 
mounted  in  the  bulb  as,  for  instance,  indicated  in  Fig.  152,  some 
observations  of  interest  were  made. 

It  was  found,  among  other  things,  that  in  such  cases,  no  mat- 
ter where  the  bombardment  began,  just  as  soon  as  a  high  tem- 
perature was  reached  there  was  generally  one  of  the  bodies 
which  seemed  to  take  most  of  the  bombardment  upon  itself,  the 
other,  or  others,  being  thereby  relieved.  The  quality  appeared 
to  depend  principally  on  the  point  of  fusion,  and  on  the  facility 
with  which  the  body  was  "  evaporated,"  or,  generally  speaking, 
disintegrated — meaning  by  the  latter  term  not  only  the  throwing 
off  of  atoms,  but  likewise  of  large  lumps.  The  observation  made 
was  in  accordance  with  generally  accepted  notions.  In  a  highly 
exhausted  bulb,  electricity  is  carried  off  from  the  electrode  by 
independent  carriers,  which  are  partly  the  atoms,  or  molecules, 
of  the  residual  atmosphere,  and  partly  the  atoms,  molecules,  or 
lumps  thrown  off  from  the  electrode.  If  the  electrode  is  com- 
posed of  bodies  of  different  character,  and  if  one  of  these  is  more 
easily  disentegrated  than  the  other,  most  of  the  electricity  sup- 
plied is  carried  off  from  that  body,  which  is  then  brought  to  a 
higher  temperature  than  the  others,  and  this  the  more,  as  upon 
an  increase  of  the  temperature  the  body  is  still  more  easily  dis- 
intregrated. 


HIGH  FREQUENCY  AND  HIGH  POTENTIAL  CURRENTS.      249 

It  seems  to  me  quite  probable  that  a  similar  process  takes  place 
in  the  bulb  even  with  a  homogeneous  electrode,  and  I  think  it 
to  be  the  principal  cause  of  the  disintegration.  There  is  bound 
to  be  some  irregularity,  even  if  the  surface  is  highly  polished, 
which,  of  course,  is  impossible  with  most  of  the  refractory  bodies 
employed  as  electrodes.  Assume  that  a  point  of  the  electrode 
gets  hotter ;  instantly  most  of  the  discharge  passes  through  that 
point,  and  a  minute  patch  it  probably  fused  and  evaporated.  It 
is  now  possible  that  in  consequence  of  the  violent  disintegration 
the  spot  attacked  sinks  in  temperature,  or  that  a  counter  force  is 
created,  as  in  an  arc  ;  at  any  rate,  the  local  tearing  off  meets  with 
the  limitations  incident  to  the  experiment,  whereupon  the  same 
process  occurs  on  another  place.  To  the  eye  the  electrode  ap- 
pears uniformly  brilliant,  but  there  are  upon  it  points  constantly 
shifting  and  wandering  around,  of  a  temperature  far  above  the 
mean,  and  this  materially  hastens  the  process  of  deterioration. 
That  some  such  thing  occurs,  at  least  when  the  electrode  is  at  a  lower 
temperature,  sufficient  experimental  evidence  can  be  obtained  in 
the  following  manner :  Exhaust  a  bulb  to  a  very  high  degree,  so 
that  with  a  fairly  high  potential  the  discharge  cannot  pass — that 
is,  not  a  luminous  one,  for  a  weak  invisible  discharge  occurs 
always,  in  all  probability.  Now  raise  slowly  and  carefully  the 
potential,  leaving  the  primary  current  on  no  more  than  for  an 
instant.  At  a  certain  point,  two,  three,  or  half  a  dozen  phos- 
phorescent spots  will  appear  on  the  globe.  These  places  of  the 
glass  are  evidently  more  violently  bombarded  than  others,  this 
being  due  to  the  unevenly  distributed  electric  density,  necessi- 
tated, of  course,  by  sharp  projections,  or,  generally  speaking,  ir- 
regularities of  the  •  electrode.  But  the  luminous  patches  are 
constantly  changing  in  position,  which  is  especially  well  observ- 
able if  one  manages  to  produce  very  few,  and  this  indicates  that 
the  configuration  of  the  electrode  is  rapidly  changing. 

From  experiences  of  this  kind  I  am  led  to  infer  that,  in  order 
to  be  most  durable,  the  refractory  button  in  the  bulb  should  be 
in  the  form  of  a  sphere  with  a  highly  polished  surface.  Such  a 
small  sphere  could  be  manufactured  from  a  diamond  or  some 
other  crystal,  but  a  better  way  would  be  to  fuse,  by  the  employ- 
ment of  extreme  degrees  of  temperature,  some  oxide — as,  fo 
instance,  zirconia — into  a  small  drop,  and  then  keep  it  in  the 
bulb  at  a  temperature  somewhat  below  its  point  of  fusion. 

Interesting  and  useful  results  can,  no  doubt,  be  reached  in  the 


250  INVENTIONS  OF  NIKOLA  TESLA. 

direction  of  extreme  degrees  of  heat.  How  can  such  high  tem- 
peratures he  arrived  at  ?  How  are  the  highest  degrees  of  heat 
readied  in  nature  ?  By  the  impact  of  stars,  by  high  speeds  and 
collisions.  In  a  collision  any  rate  of  heat  generation  may  be 
attained.  In  a  chemical  process  we  are  limited.  When  oxygen 
and  hydrogen  combine,  they  fall,  metaphorically  speaking,  from 
a  definite  height.  We  cannot  go  very  far  with  a  blast,  nor  by 
confining  heat  in  a  furnace,  but  in  an  exhausted  bulb  we  can 
concentrate  any  amount  of  energy  upon  a  minute  button.  Leav- 
ing practicability  out  of  consideration,  this,  then,  would  be  the 
means  which,  in  my  opinion,  would  enable  us  to  reach  the  highest 
temperature.  But  a  great  difficulty  when  proceeding  in  this  way 
is  encountered,  namely,  in  most  cases  the  body  is  carried  off  be- 
fore it  can  fuse  and  form  a  drop.  This  difficulty  exists  princip- 
ally with  an  oxide,  such  as  zirconia,  because  it  cannot  be  com- 
pressed in  so  hard  a  cake  that  it  would  not  be  carried  off  quickly. 
I  have  endeavored  repeatedly  to  fuse  zirconia,  placing  it  in  a  cup  of 
arc  light  carbon,  as  indicated  in  Fig.  152.  It  glowed  with  a  most 
intense  light,  and  the  stream  of  the  particles  projected  out  of  the 
carbon  cup  was  of  a  vivid  white ;  but  whether  it  was  compressed 
in  a  cake  or  made  into  a  paste  with  carbon,  it  was  carried  off 
before  it  could  be  fused.  The  carbon  cup,  containing  zirconia, 
had  to  be  mounted  very  low  in  the  neck  of  a  large  bulb,  as  the 
heating  of  the  glass  by  the  projected  particles  of  the  oxide  was 
so  rapid  that  in  the  first  trial  the  bulb  was  cracked  almost  in  an 
instant,  when  the  current  was  turned  on.  The  heating  of  the 
glass  by  the  projected  particles  was  found  to  be  always  greater 
when  the  carbon  cup  contained  a  body  which  was  rapidly  carried 
off — I  presume,  because  in  such  cases,  with  the  same  potential, 
higher  speeds  were  reached,  and  also  because,  per  unit  of  time, 
more  matter  was  projected — that  is,  more  particles  would  strike 
the  glass. 

The  before-mentioned  difficulty  did  not  exist,  however,  when 
the  body  mounted  in  the  carbon  cup  offered  great  resistance  to 
deterioration.  For  instance,  when  an  oxide  was  first  fused  in 
an  oxygen  blast,  and  then  mounted  in  the  bulb,  it  melted  very 
readily  into  a  drop. 

Generally,  during  the  process  of  fusion,  magnificent  light 
effects  were  noted,  of  which  it  would  be  difficult  to  give  an  ade- 
quate idea.  Fig.  152  is  intended  to  illustrate  the  effect  observed 
with  a  ruby  drop.  At  first  one  may  see  a  narrow  funnel  of 


man  FREQUENCY  AND  HTGH  POTENTIAL  CURRENTS    251 

white  light  projected  against  the  top  of  the  globe,  where  it 
produces  an  irregularly  outlined  phosphorescent  patch.  When  the 
point  of  the  ruby  fuses,  the  phosphorescence  becomes  very  power- 
ful ;  but  as  the  atoms  are  projected  with  much  greater  speed 
from  the  surface  of  the  drop,  soon  the  glass  gets  hot  and  "tired," 
and  now  only  the  outer  edge  of  the  patch  glows.  In  this  manner 
an  intensely  phosphorescent,  sharply  defined  line,  £,  correspond- 
ing to  the  outline  of  the  drop,  is  produced,  which  spreads  slowly 
over  the  globe  as  the  drop  gets  larger.  When  the  mass  begins 
to  boil,  small  bubbles  and  cavities  are  formed,  which  cause  dark 
colored  spots  to  sweep  across  the  globe.  The  bulb  may  be 
turned  downward  without  fear  of  the  drop  falling  off,  as  the 
mass  possesses  considerable  viscosity. 

I  may  mention  here  another  feature  of  some  interest,  which 
I  believe  to  have  noted  in  the  course  of  these  experiments, 
though  the  observations  do  not  amount  to  a  certitude.  It  ap- 
peared that  under  the  molecular  impact  caused  by  the  rapidly 
alternating  potential,  the  body  was  fused  and  maintained  in  that 
state  at  a  lower  temperature  in  a  highly  exhausted  bulb  than 
was  the  case  at  normal  pressure  and  application  of  heat  in  the 
ordinary  way — that  is,  at  least,  judging  from  the  quantity  of  the 
light  emitted.  One  of  the  experiments  performed  may  be  men- 
tioned here  by  way  of  illustration.  A  small  piece  of  pumice 
stone  was  stuck  on  a  platinum  wire,  and  first  melted  to  it  in  a 
gas  burner.  The  wire  was  next  placed  between  two  pieces  of 
charcoal,  and  a  burner  applied,  so  as  to  produce  an  intense  heat, 
sufficient  to  melt  down  the  pumice  stone  into  a  small  glass-like 
button.  The  platinum  wire  had  to  be  taken  of  sufficient  thick- 
ness, to  prevent  its  melting  in  the  fire.  While  in  the  charcoal 
fire,  or  when  held  in  a  burner  to  get  a  better  idea  of  the  degree 
of  heat,  the  button  glowed  with  great  brilliancy.  The  wire  with 
the  button  was  then  mounted  in  a  bulb,  and  upon  exhausting  the 
same  to  a  high  degree,  the  current  was  turned  on  slowly,  so  as  to 
prevent  the  cracking  of  the  button.  The  button  was  heated  to 
the  point  of  fusion,  and  when  it  melted,  it  did  not,  apparently, 
glow  with  the  same  brilliancy  as  before,  and  this  would  indicate 
a  lower  temperature.  Leaving  out  of  consideration  the  observ- 
er's possible,  and  even  probable,  error,  the  question  is,  can  a  body 
under  these  conditions  be  brought  from  a  solid  to  a  liquid  state 
with  the  evolution  of  less  light  ? 

When  the  potential  of  a  body  is  rapidly  alternated,  it  is  certain 


252 .  INVENTIONS  OF  NIKOLA  TESLA. 

that  the  structure  is  jarred.  When  the  potential  is  very  high, 
although  the  vibrations  may  be  few — say  20,000  per  second — the 
effect  upon  the  structure  may  be  considerable.  Suppose,  for  ex- 
ample, that  a  ruby  is  melted  into  a  drop  by  a  steady  application" 
of  energy.  When  it  forms  a  drop,  it  will  emit  visible  and  in- 
visible waves,  which  will  be  in  a  definite  ratio,  and  to  the  eye  the 
drop  will  appear  to  be  of  a  certain  brilliancy.  Next,  suppose  we 
diminish  to  any  degree  we  choose  the  energy  steadily  supplied, 
and,  instead,  supply  energy  which  rises  and  falls  according  to  a 
certain  law.  Now,  when  the  drop  is  formed,  there  will  be  emit- 
ted from  it  three  different  kinds  of  vibrations — the  ordinary 
visible,  and  two  kinds  of  invisible  waves :  that  is,  the  ordinary 
dark  waves  of  all  lengths,  and,  in  addition,  waves  of  a  well  de- 
fined character.  The  latter  would  not  exist  by  a  steady  supply 
of  the  energy  ;  still  they  help  to  jar  and  loosen  the  structure.  If 
this  really  be  the  case,  then  the  ruby  drop  will  emit  relatively 
less  visible  and  more  invisible  waves  than  before.  Thus  it  would 
seem  that  when  a  platinum  wire,  for  instance,  is  fused  by  currents 
alternating  with  extreme  rapidity,  it  emits  at  the  point  of  fusion 
less  light  and  more  ..visible  radiation  than  it  does  when  melted  by 
a  steady  current,  though  the  total  energy  used  up  in  the  process 
of  fusion  is  the  same  in  both  cases.  Or,  to  cite  another  example, 
a  lamp  filament  is  not  capable  of  withstanding  as  long  with  cur- 
rents of  extreme  frequency  as  it  does  with  steady  currents, 
assuming  that  it  be  worked  at  the  same  luminous  intensity.  This 
means  that  for  rapidly  alternating  currents  the  filament  should 
be  shorter  and  thicker.  The  higher  the  frequency — that  is,  the 
greater  the  departure  from  the  steady  flow — the  worse  it  would 
be  for  the  filament.  But  if  the  truth  of  this  remark  were  de- 
monstrated, it  would  be  erroneous  to  conclude  that  such  a  refrac- 
tory button  as  used  in  these  bulbs  would  be  deteriorated  quicker 
by  currents  of  extremely  high  frequency  than  by  steady  or  low 
frequency  currents.  From  experience  I  may  say  that  just  the 
opposite  holds  good :  the  button  withstands  the  bombardment 
better  with  currents  of  very  high  frequency.  But  this  is  due  to 
the  fact  that  a  high  frequency  discharge  passes  through  a  rarefied 
gas  with  much  greater  freedom  than  a  steady  or  low  frequency 
discharge,  and  this  will  mean  that  with  the  former  we  can  work 
with  a  lower  potential  or  with  a  less  violent  impact.  As  long, 
then,  as  the  gas  is  of  no  consequence,  a  steady  or  low  frequency 
current  is  better ;  but  as  soon  as  the  action  of  the  gas  is  desired 
and  important,  high  frequencies  are  preferable. 


HIGH  FREQUENCY  AND  HIGH  POTENTIAL  CURRENTS.    253 

In  the  course  of  these  experiments  a  great  many  trials  were 
made  with  all  kinds  of  carbon  buttons.  Electrodes  made  of  or- 
dinary carbon  buttons  were  decidedly  more  durable  when  the 
buttons  were  obtained  by  the  application  of  enormous  pressure. 
Electrodes  prepared  by  depositing  carbon  in  well  known  ways 
did  not  show  up  well ;  they  blackened  the  globe  very  quickly. 
From  many  experiences  I  conclude  that  lamp  filaments  obtained 
in  this  manner  can  be  advantageously  used  only  with  low  poten- 
tials and  low  frequency  currents.  Some  kinds  of  carbon  withstand 
so  well  that,  in  order  to  bring  them  to  the  point  of  fusion,  it  is 
necessary  to  employ  very  small  buttons.  In  this  case  the  obser- 
vation is  rendered  very  difficult  on  account  of  the  intense  heat 
produced.  Nevertheless  there  can  be  no  doubt  that  all  kinds  of 
carbon  are  fused  under  the  molecular  bombardment,  but  the 
liquid  state  must  be  one  of  great  instability.  Of  all  the  bodies 
tried  there  were  two  which  withstood  best — diamond  and  car- 
borundum. These  two  showed  up  about  equally,  but  the  latter 
was  preferable  for  many  reasons.  As  it  is  more  than  likely  that 
this  body  is  not  yet  generally  known,  I  Avill  venture  to  call  your 
attention  to  it. 

It  has  been  recently  produced  by  Mr.  E.  G.  Acheson,  of 
Monongahela  City,  Pa.,  II.  S.  A.  It  is  intended  to  replace  ordi- 
nary diamond  powder  for  polishing  precious  stones,  etc.,  and  I 
have  been  informed  that  it  accomplishes  this  object  quite  suc- 
cessfully. I  do  not  know  why  the  name  "  carborundum "  has 
been  given  to  it,  unless  there  is  something  in  the  process  of  its 
manufacture  which  justifies  this  selection.  Through  the  kindness 
of  the  inventor,  I  obtained  a  short  while  ago  some  samples  which 
I  desired  to  test  in  regard  to  their  qualities  of  phosphorescence 
and  capability  of  withstanding  high  degrees  of  heat. 

Carborundum  can  be  obtained  in  two  forms — in  the  form  of 
"crystals"  and  of  powder.  The  former  appear  to  the  naked  eye 
dark  colored,  but  are  very  brilliant ;  the  latter  is  of  nearly  the 
same  color  as  ordinary  diamond  powder,  but  very  much  finer. 
When  viewed  under  a  microscope  the  samples  of  crystals  given 
to  me  did  not  appear  to  have  any  definite  form,  but  rather  re- 
sembled pieces  of  broken  up  egg  coal  of  fine  quality.  The 
majority  were  opaque,  but  there  were  some  which  were  trans- 
parent and  colored.  The  crystals  are  a  kind  of  carbon  containing 
some  impurities ;  they  are  extremely  hard,  and  withstand  for  a 
long  time  even  an  oxygen  blast.  When  the  blast  is  directed 


254  INVENTIONS  OF  NIKOLA  TESLA. 

against  them  they  at  first  form  a  cake  of  some  compactness,  prob- 
ably in  consequence  of  the  fusion  of  impurities  they  contain.  The 
mass  withstands  for  a  very  long  time  the  blast  without  further 
fusion ;  but  a  slow  carrying  off,  or  burning,  occurs,  and,  finally, 
a  small  quantity  of  a  glass-like  residue  is  left,  wrhich,  I  suppose, 
is  melted  alumina.  When  compressed  strongly  they  conduct  very 
well,  but  not  as  well  as  ordinary  carbon.  The  powder,  which  is 
obtained  from  the  crystals  in  some  way,  is  practically  non-con- 
ducting. It  affords  a  magnificent  polishing  material  for  stones. 

The  time  has  been  too  short  to  make  a  satisfactory  study  of 
the  properties  of  this  product,  but  enough  experience  has  been 
gained  in  a  few  weeks  I  have  experimented  upon  it  to  say  that 
it  does  possess  some  remarkable  properties  in  many  respects.  It 
withstands  excessively  high  degrees  of  heat,  it  is  little  deteriorated 
by  molecular  bombardment,  and  it  does  not  blacken  the  globe  as 
ordinary  carbon  does.  The  only  difficulty  which  I  have  experienced 
in  its  use  in  connection  with  these  experiments  was  to  find  some 
binding  material  which  would  resist  the  heat  and  the  effect  of  the 
bombardment  as  successfully  as  carborundum  itself  does. 

I  have  here  a  number  of  bulbs  which  I  have  provided  with 
buttons  of  carborundum.  To  make  such  a  button  of  carborun- 
dum crystals  I  proceed  in  the  following  manner:  I  take  an  or- 
dinary lamp  filament  and  dip  its  point  in  tar,  or  some  other 
thick  substance  or  paint  which  may  be  readily  carbonized.  I 
next  pass  the  point  of  the  filament  through  the  crystals,  and  then 
hold  it  vertically  over  a  hot  plate.  The  tar  softens  and  forms  a 
drop  on  the  point  of  the  filament,  the  crystals  adhering  to  the 
surface  of  the  drop.  By  regulating  the  distance  from  the  plate 
the  tar  is  slowly  dried  out  and  the  button  becomes  solid.  I  then 
once  more  dip  the  button  in  tar  and  hold  it  again  over  a  plate 
until  the  tar  is  evaporated,  leaving  only  a  hard  mass  which  firmly 
binds  the  crystals.  When  a  larger  button  is  required  I  repeat 
the  process  several  times,  and  I  generally  also  cover  the  filament 
a  certain  distance  below  the  button  with  crystals.  The  button 
being  mounted  in  a  bulb,  when  a  good  vacuum  has  been  reached, 
first  a  weak  and  then  a  strong  discharge  is  passed  through  the 
bulb  to  carbonize  the  tar  and  expel  all  gases,  and  later  it  is  brought 
to  a  very  intense  incandescence. 

When  the  powder  is  used  I  have  found  it  best  to  proceed  as 
follows  :  I  make  a  thick  paint  of  carborundum  and  tar,  and  pass 
a  lamp  filament  through  the  paint.  Taking  then  most  of  the 


UIGU  FREQUENCY  AND  HIGH  POTENTIAL  CURRENTS.     255 

paint  off  by  rubbing  the  filament  against  a  piece  of  chamois 
leather,  I  hold  it  over  a  hot  plate  until  the  tar  evaporates  and  the 
coating  becomes  firm.  I  repeat  this  process  as  many  times  as  it 
is  necessary  to  obtain  a  certain  thickness  of  coating.  On  the 
point  of  the  coated  filament  I  form  a  button  in  the  same 
manner. 

There  is  no  doubt  that  such  a  button — properly  prepared  under 
great  pressure — of  carborundum,  especially  of  powder  of  the  best 
quality,  will  withstand  the  effect  of  the  bombardment  fully  as 
well  as  anything  we  know.  The  difficulty  is  that  the  binding 
material  gives  way,  and  the  carborundum  is  slowly  thrown  off 
after  some  time.  As  it  does  not  seem  to  blacken  the  globe  in  the 
least,  it  might  be  found  useful  for  coating  the  filaments  of  ordinary 
incandescent  lamps,  and  I  think  that  it  is  even  possible  to  produce 
thin  threads  or  sticks  of  carborundum  which  will  replace  the  or- 
dinary filaments  in  an  incandescent  lamp.  A  carborundum  coat- 
ing seems  to  be  more  durable  than  other  coatings,  not  only 
because  the  carborundum  can  withstand  high  degrees  of  heat,  but 
also  because  it  seems  to  unite  with  the  carbon  better  than  any 
other  material  I  have  tried.  A  coating  of  zirconia  or  any  other 
oxide,  for  instance,  is  far  more  quickly  destroyed.  I  prepared 
buttons  of  diamond  dust  in  the  same  manner  as  of  carborundum, 
and  these  came  in  durability  nearest  to  those  prepared  of  car- 
borundum, but  the  binding  paste  gave  way  much  more  quickly 
in  the  diamond  buttons ;  this,  however,  I  attributed  to  the  size 
and  irregularity  of  the  grains  of  the  diamond. 

It  was  of  interest  to  find  whether  carborundum  possesses  the 
quality  of  phosphorescence.  One  is,  of  course,  prepared  to  en- 
counter two  difficulties :  first,  as  regards  the  rough  product,  the 
"crystals,"  they  are  good  conducting,  and  it  is  a  fact  that  con- 
ductors do  not  phosphoresce;  second,  the  powder,  being  exceed- 
ingly fine,  would  not  be  apt  to  exhibit  very  prominently  this 
quality,  since  we  know  that  when  crystals,  even  such  as  diamond 
or  ruby,  are  finely  powdered,  they  lose  the  property  of  phos- 
phorescence to  a  considerable  degree. 

The  question  presents  itself  here,  can  a  conductor  phosphor- 
esce ?  What  is  there  in  such  a  body  as  a  metal,  for  instance,  that 
would  deprive  it  of  the  quality  of  phosphoresence,  unless  it  is 
that  property  which  characterizes  it  as  a  conductor  ?  For  it  is  a 
fact  that  most  of  the  phosphorescent  bodies  lose  that  quality  when 
they  are  sufficiently  heated  to  become  more  or  less  conducting. 


256  INVENTIONS  OF  NIKOLA  TESLA. 

Then,  if  a  metal  be  in  a  large  measure,  or  perhaps  entirely,  de- 
prived of  that  property,  it  should  be  capable  of  phosphoresence. 
Therefore  it  is  quite  possible  that  at  some  extremely  high  fre- 
quency, when  behaving  practically  as  a  non-conductor,  a  metal 
or  any  other  conductor  might  exhibit  the  quality  of  phosphores- 
ence, even  though  it  be  entirely  incapable  of  phosphorescing 
under  the  impact  of  a  low-frequency  discharge.  There  is,  how- 
ever, another  possible  way  how  a  conductor  might  at  least  appear 
to  phosphoresce. 

Considerable  doubt  still  exists  as  to  what  really  is  phosphor- 
escence, and  as  to  whether  the  various  phenomena  comprised 
under  this  head  are  due  to  the  same  causes.  Suppose  that  in  an 
exhausted  bulb,  under  the  molecular  impact,  the  surface  of  a 
piece  of  metal  or  other  conductor  is  rendered  strongly  luminous, 
but  at  the  same  time  it  is  found  that  it  remains  comparatively 
cool,  would  not  this  luminosity  be  called  phosphorescence?  Now 
such  a  result,  theoretically  at  least,  is  possible,  for  it  is  a  mere 
question  of  potential  or  speed.  Assume  the  potential  of  the 
electrode,  and  consequently  the  speed  of  the  projected  atoms,  to 
be  sufficiently  high,  the  surface  of  the  metal  piece,  against  which 
the  atoms  are  projected,  would  be  rendered  highly  incandescent, 
since  the  process  of  heat  generation  would  be  incomparably  faster 
than  that  of  radiating  or  conducting  away  from  the  surface  of 
the  collision.  In  the  eye  of  the  observer  a  single  impact  of  the 
atoms  would  cause  an  instantaneous  flash,  but  if  the  impacts  were 
repeated  with  sufficient  rapidity,  they  would  produce  a  continu- 
ous impression  upon  his  retina.  To  him  then  the  surface  of  the 
metal  would  appear  continuously  incandescent  and  of  constant 
luminous  intensity,  while  in  reality  the  light  would  be  either 
intermittent,  or  at  least  changing  periodically  in  intensity.  The 
metal  piece  would  rise  in  temperature  until  equilibrium  was 
attained — that  is,  until  the  energy  continuously  radiated  would 
equal  that  intermittently  supplied.  But  the  supplied  energy 
might  under  such  conditions  not  be  sufficient  to  bring  the  body 
to  any  more  than  a  very  moderate  mean  temperature,  especially 
if  the  frequency  of  the  atomic  impacts  be  very  low — just  enough 
that  the  fluctuation  of  the  intensity  of  the  light  emitted  could 
not  be  detected  by  the  eye.  The  body  would  now,  owing  to  the 
manner  in  which  the  energy  is  supplied,  emit  a  strong  light,  and 
yet  be  at  a  comparatively  very  low  mean  temperature.  How 
should  the  observer  name  the  luminosity  thus  produced  ?  Even  if 


HIGH  FREQUENCY  AND  HIGH  POTENTIAL  CURRENTS.      257 

the  analysis  of  the  light  would  teach  him  something  definite,  still 
he  would  probably  rank  it  under  the  phenomena  of  phosphor- 
escence. It  is  conceivable  that  in  such  a  way  both  conducting 
and  non-conducting  bodies  may  be  maintained  at  a  certain  lumin- 
ous intensity,  but  the  energy  required  would  very  greatly  vary 
with  the  nature  and  properties  of  the  bodies. 

These  and  some  foregoing  remarks  of  a  speculative  nature 
were  made  merely  to  bring  out  curious  features  of  alternate 
currents  or  electric  impulses.  By  their  help  we  may  cause  a  body 
to  emit  more  light,  while  at  a  certain  mean  temperature,  than  it 
would  emit  if  brought  to  that  temperature  by  a  steady  supply ; 
and,  again,  we  may  bring  a  body  to  the  point  of  fusion,  and  cause 
it  to  emit  less  light  than  when  fused  by  the  application  of  energy 
in  ordinary  ways.  It  all  depends  on  how  we  supply  the  energy, 
and  what  kind  of  vibrations  we  set  up ;  in  one  case  the  vibrations 
are  more,  in  the  other  less,  adapted  to  affect  our  sense  of  vision. 

Some  effects,  which  I  had  not  observed  before,  obtained  with 
carborundum  in  the  first  trials,  I  attributed  to  phosphorescence, 
but  in  subsequent  experiments  it  appeared  that  it  was  devoid  of 
that  quality.  The  crystals  possess  a  noteworthy  feature.  In  a 
bulb  provided  with  a  single  electrode  in  the  shape  of  a  small 
circular  metal  disc,  for  instance,  at  a  certain  degree  of  exhaustion 
the  electrode  is  covered  with  a  milky,  film,  which  is  separated  by 
a  dark  space  from  the  glow  filling  the  bulb.  When  the  metal 
disc  is  covered  with  carborundum  crystals,  the  film  is  far  more 
intense,  and  snow-white.  This  I  found  later  to  be  merely  an 
effect  of  the  bright  surface  of  the  crystals,  for  when  an  aluminum 
electrode  was  highly  polished,  it  exhibited  more  or  less  the  same 
phenomenon.  I  made  a  number  of  experiments  with  the  samples 
of  crystals  obtained,  principally  because  it  would  have  been  of 
special  interest  to  find  that  they  are  capable  of  phosphorescence, 
on  account  of  their  being  conducting.  I  could  not  produce  phos- 
phorescence distinctly,  but  I  must  remark  that  a  decisive  opinion 
cannot  be  formed  until  other  experimenters  have  gone  over  the 
same  ground. 

The  powder  behaved  in  some  experiments  as  though  it  con- 
tained alumina,  but  it  did  not  exhibit  with  sufficient  distinctness 
the  red  of  the  latter.  Its  dead  color  brightens  considerably  un- 
der the  molecular  impact,  but  I  am  now  convinced  it  does  not 
phosphoresce.  Still,  the  tests  with  the  powder  are  not  conclusive, 
because  powdered  carborundum  probably  does  not  behave  like  a 


258  INVENTIONS  OF  NIKOLA  TESLA. 

phosphorescent  sulphide,  for  example,  which  could  be  finely 
powdered  without  impairing  the  phosphorescence,  but  rather  like 
powdered  ruby  or  diamond,  and  therefore  it  would  be  necessary, 
in  order  to  make  a  decisive  test,  to  obtain  it  in  a  large  lump  and 
polish  up  the  surface. 

If  the  carborundum  proves  useful  in  connection  •with  these 
and  similar  experiments,  its  chief  value  will  be  found  in  the 
production  of  coatings,  thin  conductors,  buttons,  or  other  elec- 
trodes capable  of  withstanding  extremely  high  degrees  of  heat. 

The  production  of  a  sniall  electrode,  capable  of  wit hstan< lino- 
enormous  temperatures,  I  regard  as  of  the  greatest  importance 
in  the  manufacture  of  light.  It  would  enable  us  to  obtain,  by 
means  of  currents  of  very  high  frequencies,  certainly  20  times,  if 
not  more,  the  quantity  of  light  which  is  obtained  in  the  present 
incandescent  lamp  by  the  same  expenditure  of  energy.  This 
estimate  may  appear  to  many  exaggerated,  but  in  reality  I  think 
it  is  far  from  being  so.  As  this  statement  might  be  misunder- 
stood, I  think  it  is  necessary  to  expose  clearly  the  problem  with 
which,  in  this  line  of  work,  we  are  confronted,  and  the  manner 
in  which,  in  my  opinion,  a  solution  will  be  arrived  at. 

Any  one  who  begins  a  study  of  the  problem  will  be  apt  to 
think  that  what  is  wanted  in  a  lamp  with  an  electrode  is  a  very 
high  degree  of  incandescence  of  the  electrode.  There  he  will  be 
mistaken.  The  high  incandescence  of  the  button  is  a  necessary 
evil,  but  what  is  really  wanted  is  the  high  incandescence  of  the 
gas  surrounding  the  button.  In  other  words,  the  problem  in 
such  a  lamp  is  to  bring  a  mass  of  gas  to  the  highest,  possible  in- 
candescence. The  higher  the  incandescence,  the  quicker  the 
mean  vibration,  the  greater  is  the  economy  of  the  light  production. 
But  to  maintain  a  mass  of  gas  at  a  high  degree  of  incandescence 
in  a  glass  vessel,  it  will  always  be  necessary  to  keep  the  incande- 
scent mass  away  from  the  glass ;  that  is,  to  confine  it  as  much  as 
possible  to  the  central  portion  of  the  globe. 

In  one  of  the  experiments  this  evening  a  brush  was  produced 
at  the  end  of  a  wire.  The  brush  was  a  flame,  a  source  of  heat 
and  light.  It  did  not  emit  much  perceptible  heat,  nor  did  it 
glow  with  an  intense  light ;  but  is  it  the  less  a  flame  because  it 
does  not  scorch  my  hand  {  Is  it  the  less  a  flame  because  it  does 
not  hurt  my  eyes  by  its  brilliancy  ?  The  problem  is  precisely  to 
produce  in  the  bulb  such  a  flame,  much  smaller  in  size,  but  in- 
comparably more  powerful.  Were  there  means  at  hand  for 


HIGH  FREQVENCF  AND  HIGH  POTENTIAL  CURRENTS.     259 

producing  electric  impulses  of  a  sufficiently  high  frequency,  and 
for  transmitting  them,  the  bulb  could  be  done  away  with,  unless 
it  were  used  to  protect  the  electrode,  or  to  economize  the  energy 
by  confining  the  heat.  But  as  such  means  are  not  at  disposal,  it 
becomes  necessary  to  place  the  terminal  in  the  bulb  and  rarefy 
the  air  in  the  same.  This  is  done  merely  to  enable  the  apparatus 
to  perform  the  work  which  it  is  not  capable  of  performing  at  or- 
dinary air  pressure.  In  the  bulb  we  are  able  to  intensify  the 
action  to  any  degree — so  far  that  the  brush  emits  a  powerful 
light. 

The  intensity  of  the  light  emitted  depends  principally  on  the 
frequency  and  potential  of  the  impulses,  and  on  the  electric  den- 
sity on  the  surface  of  the  electrode.  It  is  of  the  greatest  impor- 
tance to  employ  the  smallest  possible  button,  in  order  to  push 
the  density  very  far.  Under  the  violent  impact  of  the  molecules 
of  the  gas  surrounding  it,  the  small  electrode  is  of  course  brought 
to  an  extremely  high  temperature,  but  around  it  is  a  mass  of 
highly  incandescent  gas,  a  flame  photosphere,  many  hundred 
times  the  volume  of  the  electrode.  With  a  diamond,  carborun- 
dum or  zirconia  button  the  photosphere  can  be  as  much  as  one 
thousand  times  the  volume  of  the  button.  Without  much  re- 
flection one  would  tl link  that  in  pushing  so  far  the  incandescence 
of  the  electrode  it  would  be  instantly  volatilized.  But  after  a 
careful  consideration  one  would  find  that,  theoretically,  it  should 
not  occur,  and  in  this  fact — which,  moreover,  is  experimentally 
demonstrated — lies  principally  the  future  value  of  such  a  lamp. 

At  first,  when  the  bombardment  begins,  most  of  the  work  is 
performed  on  the  surface  of  the  button,  but  when  a  highly  con- 
ducting photosphere  is  formed  the  button  is  comparatively  re- 
lieved. The  higher  the  incandescence  of  the  photosphere,  the 
more  it  approaches  in  conductivity  to  that  of  the  electrode,  and 
the  more,  therefore,  the  solid  and  the  gas  form  one  conducting 
body.  The  consequence  is  that  the  further  the  incandescence  is 
forced  the  more  work,  comparatively,  is  performed  on  the  gas, 
and  the  less  on  the  electrode.  The  formation  of  a  powerful 
photosphere  is  consequently  the  very  means  for  protecting  the 
electrode.  This  protection,  of  course,  is  a  relative  one,  and  it 
should  not  be  thought  that  by  pushing  the  incandescence  higher 
the  electrode  is  actually  less  deteriorated.  Still,  theoretically, 
with  extreme  frequencies,  this  result  must  be  reached,  but  prob- 
ably at  a  temperature  too  high  for  most  of  the  refractory  bodies 


360  INVENTIONS  OF  NIKOLA  TEFL  A. 

known.  Given,  then,  an  electrode  which  can  withstand  to  a  very 
high  limit  the  effect  of  the  bombardment  and  outward  strain,  it 
would  be  safe,  no  matter  how  much  it  was  forced  beyond  that 
limit.  In  an  incandescent  lamp  quite  different  considerations 
apply.  There  the  gas  is  not  at  all  concerned  ;  the  whole  of  the 
work  is  performed  on  the  filament ;  and  the  the  life  of  the  lamp 
diminishes  so  rapidly  with  the  increase  of  the  degree  of  incan- 
descence that  economical  reasons  compel  us  to  work  it  at  a  low 
incandescence.  But  if  an  incandescent  lamp  is  operated  with 
currents  of  very  high  frequency,  the  action  of  the  gas  cannot  be 
neglected,  and  the  rules  for  the  most  economical  working  must 
be  considerably  modified. 

In  order  to  bring  such  a  lamp  with  one  or  two  electrodes  to  a 
great  perfection,  it  is  necessary  to  employ  impulses  of  very  high 
frequency.  The  high  frequency  secures,  among  others,  two  chief 
advantages,  which  have  a  most  important  bearing  upon  the 
economy  of  the  light  production.  First,  the  deterioration  of  the 
electrode  is  reduced  by  reason  of  the  fact  that  we  employ  a  great 
many  small  impacts,  instead  of  a  few  violent  ones,  which  quickly 
shatter  the  structure  ;  secondly,  the  formation  of  a  large  photo- 
shere  is  facilitated. 

In  order  to  reduce  the  deterioration  of  the  electrode  to  the 
minimum,  it  is  desirable  that  the  vibration  be  harmonic,  for  any 
suddenness  hastens  the  process  of  destruction.  An  electrode  lasts 
much  longer  when  kept  at  incandescence  by  currents,  or  impulses, 
obtained  from  a  high  frequency  alternator,  which  rise  and  fall 
more  or  less  harmonically,  than  by  impulses  obtained  from  a  dis- 
ruptive discharge  coil.  In  the  latter  case  there  is  no  doubt  that 
most  of  the  damage  is  done  by  the  fundamental  sudden  dis- 
charges. 

One  of  the  elements  of  loss  in  such  a  lamp  is  the  bombard- 
ment of  the  globe.  As  the  potential  is  very  high,  the  molecules 
are  pro  jected  with  great  speed ;  they  strike  the  glass,  and  usually  ex- 
cite a  strong  phosphorescence.  The  effect  produced  is  very  pretty , 
but  for  economical  reasons  it  would  be  perhaps  preferable  to  pre- 
vent, or  at  least  reduce  to  a  minimum,  the  bombardment  against 
the  globe,  as  in  such  case  it  is,  as  a  rule,  not  the  object  to  excite 
phosphorescence,  and  as  some  loss  of  energy  results  from  the 
bombardment.  This  loss  in  the  bulb  is  principally  dependent 
on  the  potential  of  the  impulses  and  on  the  electric  density  on 
the  surface  of  the  electrode.  In  employing  \ cry  high  frecjuen- 


HIGH  FREQUENCY  AND  HIGH  POTENTIAL  CURRENTS.      261 

cies  the  loss  of  energy  by  the  bombardment  is  greatly  reduced, 
for,  first,  the  potential  needed  to  perform  a  given  amount  of  work 
is  much  smaller ;  and,  secondly,  by  producing  a  highly  conduct- 
ting  photosphere  around  the  electrode,  the  same  result  is  obtained 
as  though  the  electrode  were  much  larger,  which  is  equivalent  to 
a  smaller  electric  density.  But  be  it  by  the  diminution  of  the 
maximum  potential  or  of  the  density,  the  gain  is  effected  in  the 
same  manner,  namely,  by  avoiding  violent  shocks,  which  strain 
the  glass  much  beyond  its  limit  of  elasticity.  If  the  frequency 
could  be  brought  high  enough,  the  loss  due  to  the  imperfect 
elasticity  of  the  glass  would  be  entirely  negligible.  The  loss  due 
to  bombardment  of  the  globe  may,  however,  be  reduced  by  using 
two  electrodes  instead  of  one.  In  such  case  each  of  the  elec- 
trodes may  be  connected  to  one  of  the  terminals ;  or  else,  if  it  is 
preferable  to  use  only  one  wire,  one  electrode  may  be  connected 
to  one  terminal  and  the  other  to  the  ground  or  to  an  insulated 
body  of  some  surface,  as,  for  instance,  a  shade  on  the  lamp.  In 
the  latter  case,  unless  some  judgment  is  used,  one  of  the  elec- 
trodes might  glow  more  intensely  than  the  other. 

But  on  the  whole  I  find  it  preferable,  when  using  such  high 
frequencies,  to  employ  only  one  electrode  and  one  connecting 
wire.  I  am  convinced  that  the  illuminating  device  of  the  near 
future  will  not  require  for  its  operation  more  than  one  lead,  and, 
at  any  rate,  it  will  have  no  leading-in  wire,  since  the  energy  re- 
quired can  be  as  well  transmitted  through  the  glass.  In  experi- 
mental bulbs  the  leading-in  wire  is  not  generally  used  on  account 
of  convenience,  as  in  employing  condenser  coatings  in  the  manner 
indicated  in  Fig.  151,  for  example,  there  is  some  difficulty  in 
titting  the  parts,  but  these  difficulties  would  not  exist  if  a  great 
many  bulbs  were  manufactured ;  otherwise  the  energy  can  be 
conveyed  through  the  glass  as  well  as  through  a  wire,  and  with 
these  high  frequencies  the  losses  are  very  small.  Such  illustrat- 
ing devices  will  necessarilly  involve  the  use  of  very  high 
potentials,  and  this,  in  the  eyes  of  practical  men,  might  be  an  ob- 
jectionable feature.  Yet,  in  reality,  high  potentials  are  not 
objectionable — certainly  not  in  the  least  so  far  as  the  safety  of 
the  devices  is  concerned. 

There  are  two  ways  of  rendering  an  electric  appliance  safe. 
One  is  to  use  low  potentials,  the  other  is  to  determine  the  dimen- 
sions of  the  apparatus  so  that  it  is  safe,  no  matter  how  high  a 
potential  is  used.  Of  the  two,  the  latter  seems  to  me  the  better 


362  INVENTIONS  OF  NIKOLA  TESLA. 

way,  for  then  the  safety  is  absolute,  unaffected  by  any  possible 
combination  of  circumstances  which  might  render  even  alow-poten- 
tial appliance  dangerous  to  life  and  property.  But  the  practical 
conditions  require  not  only  the  judicious  determination  of  the 
dimensions  of  the  apparatus ;  they  likewise  necessitate  the  em- 
ployment of  energy  of  the  proper  kind.  It  is  easy,  for  instance, 
to  construct  a  transformer  capable  of  giving,  when  operated  from 
an  ordinary  alternate  current  machine  of  low  tension,  say  50,000 
volts,  which  might  be  required  to  light  a  highly  exhausted  phos- 
phorescent tube,  so  that,  in  spite  of  the  high  potential,  it  is 
perfectly  safe,  the  shock  from  it  producing  no  inconvenience. 
Still  such  a  transformer  would  be  expensive,  and  in  itself  ineffi- 
cient; and,  besides,  what  energy  was  obtained  from  it  would  not 
be  economically  used  for  the  production  of  light.  The  economy 
demands  the  employment  of  energy  in  the  form  of  extremely  rapid 
vibrations.  The  problem  of  producing  light  has  been  likened  to 
that  of  maintaining  a  certain  high-pitcli  note  by  means  of  a  bell. 
It  should  be  said  a  barely  audible  note  ;  and  even  these  words 
xvould  not  express  it,  so  wonderful  is  the  sensitiveness  of  the  eye. 
We  may  deliver  powerful  blows  at  long  intervals,  waste  a  good 
deal  of  energy,  and  still  not  get  what  we  want ;  or  we  may  keep 
up  the  note  by  delivering  frequent  taps,  and  get  nearer  to  the 
object  sought  by  the  expenditure  of  much  less  energy.  In  the 
production  of  light,  as  far  as  the  illuminating  device  is  concerned, 
there  can  be  only  one  rule — that  is,  to  use  as  high  frequencies  as 
can  be  obtained ;  but  the  means  for  the  production  and  convey- 
ance of  impulses  of  such  character  impose,  at  present  at  least, 
great  limitations.  Once  it  is  decided  to  use  very  high  frequen- 
cies, the  return  wire  becomes  unnecessary,  and  all  the  appliances 
are  simplified.  By  the  use  of  obvious  means  the  same  result  is 
obtained  as  though  the  return  wire  were  used.  It  is  sufficient  for 
this  purpose  to  bring  in  contact  with  the  bulb,  or  merely  in  the 
vicinity  of  the  same,  an  insulated  body  of  some  surface.  The 
surface  need,  of  course,  be  the  smaller,  the  higher  the  frequency 
and  potential  used,  and  necessarily,  also,  the  higher  the  economy 
of  the  lamp  or  other  device. 

This  plan  of  working  has  been  resorted  to  on  several  occasions 
this  evening.  So,  for  instance,  when  the  incandescence  of  a 
button  was  produced  by  grasping  the  bulb  with  the  hand,  the 
body  of  the  experimenter  merely  served  to  intensify  the  action. 
The  bulb  used  was  similar  to  that  illustrated  in  Fig.  148,  and 


HIGH  FREQ  UENCY  AND  HIGH  POTENTIAL  CURRENTS.     263 


tlie  coil  was  excited  to  a  small  potential,  not  sufficient  to  bring 
the  button  to  incandescence  when  the  bull)  was  hanging  from 
the  Avire ;  and  incidentally,  in  order  to  perform  the  experiment 
in  a  more  suitable  manner,  the  button  was  taken  so  large  that  a 
perceptible  time  had  to  elapse  before,  upon  grasping  the  bulb,  it 
could  be  rendered  incandescent.  The  contact  with  the  bulb  was, 
of  course,  quite  unnecessary.  It  is  easy,  by  using  a  rather  large 
bulb  with  an  exceedingly  small  electrode,  to  adjust  the  conditions 
so  that  the  latter  is  brought  to  bright  incandescence  by  the  mere 
approach  of  the  experimenter  within  a  few  feet  of  the  bulb,  and 
that  the  incandescence  subsides  upon  his  receding. 


FIG.  153. 


FIG.  154. 


In  another  experiment,  when  phosphorescence  was  excited,  a 
similar  bulb  was  used.  Here  again,  originally,  the  potential  was 
not  sufficient  to  excite  phosphorescence  until  the  action  was  in- 
tensified— in  this  case,  however,  to  present  a  different  feature,  by 
touching  the  socket  with  a  metallic  object  held  in  the  hand.  The 
electrode  in  the  bulb  was  a  carbon  button  so  large  that  it  could 
not  be  brought  to  incandescence,  and  thereby  spoil  the  effect 
produced  by  phosphorescence. 

Again,  in  another  of  the  early  experiments,  a  bulb  was  used, 


264  INVENTIONS  OF  NIKOLA  TESLA. 

as  illustrated  in  Fig.  141.  In  this  instance,  by  touching  the  bulb 
with  one  or  two  fingers,  one  or  two  shadows  of  the  stem  inside 
were  projected  against  the  glass,  the  touch  of  the  finger  producing 
the  same  results  as  the  application  of  an  external  negative  elec- 
trode under  ordinary  circumstances. 

In  all  these  experiments  the  action  was  intensified  by  augment- 
ing the  capacity  at  the  end  of  the  lead  connected  to  the  terminal. 
As  a  rule,  it  is  not  necessary  to  resort  to  such  means,  and  would 
be  quite  unnecessary  with  still  higher  frequencies ;  but  when  it 
Is  desired,  the  bulb,  or  tube,  can  be  easily  adapted  to  the  pur- 
pose. 

In  Fig.  153,  for  example,  an  experimental  bull),  i,,  is  shown, 
which  is  provided  with  a  neck,  n,  on  the  top,  for  the  application 
of  an  external  tinfoil  coating,  which  may  be  connected  to  a  body 
of  larger  surface.  Such  a  lamp  as  illustrated  in  Fig.  154  may 
also  be  lighted  by  connecting  the  tinfoil  coating  on  the  neck  n, 
to  the  terminal,  and  the  leading-in  wire,  w,  to  an  insulated  plate. 
If  the  bulb  stands  in  a  socket  upright,  as  shown  in  the  cut,  a 
shade  of  conducting  material  may  be  slipped  in  the  neck,  n,  and 
the  action  thus  magnified. 

A  more  perfected  arrangement  used  in  some  of  these  bulbs  is 
illustrated  in  Fig.  155.  In  this  case  the  construction  of  the  bulb 
is  as  shown  and  described  before,  when  reference  was  made  to 
Fig.  148.  A  zinc  sheet,  z,  with  a  tubular  extension,  T,  is  applied 
over  the  metallic  socket,  s.  The  bulb  hangs  downward  from  the 
terminal,  t,  the  zinc  sheet,  z,  performing  the  double  office  of  in- 
tensifier  and  reflector.  The  reflector  is  separated  from  the  ter- 
minal, t,  by  an  extension  of  the  insulating  plug,  P. 

A  similar  disposition  with  a  phosphorescent  tube  is  illustrated 
in  Fig.  156.  The  tube,  T,  is  prepared  from  two  short  tubes  of 
different  diameter,  which  are  sealed  on  the  ends.  Oil  the  lower 
end  is  placed  an  inside  conducting  coating,  c,  which  connects  to 
the  wire  w.  The  wire  has  a  hook  on  the  upper  end  for  suspen- 
sion, and  passes  through  the  centre  of  the  inside  tube,  which  is 
filled  witli  some  good  and  tightly  packed  insulator.  On  the  out- 
side of  the  upper  end  of  the  tube,  T,  is  another  conducting  coat- 
ing, ol}  upon  which  is  slipped  a  metallic  reflector  z,  which  should 
be  separated  by  a  thick  insulation  from  the  end  of  wire  u\ 

The  economical  use  of  such  a  reflector  or  intensifier  would  re- 
quire that  all  energy  supplied  to  an  air  condenser  should  be  re- 
coverable, or,  in  other  words,  that  there  should  not  be  any  losses, 


HIGH  FREq  UENGY  AND  HIGIt  POTENTIAL  CURRENTS.     265 

neither  in  the  gaseous  medium  nor  through  its  action  elsewhere. 
This  is  far  from  being  so,  but,  fortunately,  the  losses  may  be  re- 
duced to  anything  desired.  A  few  remarks  are  necessary  on 
this  subject,  in  order  to  make  the  experiences  gathered  in  the 
course  of  these  investigations  perfectly  clear. 

Suppose  a  small  helix  with  many  well  insulated  turns,  as  in 
experiment  Fig.  146,  has  one  of  its  ends  connected  to  one  of  the 
terminals  of  the  induction  coil,  and  the  other  to  a  metal  plate, 
or,  for  the  sake  of  simplicity,  a  sphere,  insulated  in  space.  When 
the  coil  is  set  to  work,  the  potential  of  the  sphere  is  alternated, 
and  a  small  helix  now  behaves  as  though  its  free  end  were  con- 
nected to  the  other  terminal  of  the  induction  coil.  If  an  iron 
rod  be  held  within  a  small  helix,  it  is  quickly  brought  to  a  high 


FIG.  155. 

temperature,  indicating  the  passage  of  a  strong  current  through 
the  helix.  How  does  the  insulated  sphere  act  in  this  case  ?  It 
can  be  a  condenser,  storing  and  returning  the  energy  supplied  to 
it,  or  it  can  be  a  mere  sink  of  energy,  and  the  conditions  of  the 
experiment  determine  whether  it  is  rather  one  than  the  other. 
The  sphere  being  charged  to  a  high  potential,  it  acts  inductively 
upon  the  surrounding  air,  or  whatever  gaseous  medium  there  might 
be.  The  molecules,  or  atoms,  which  are  near  the  sphere,  are  of 
course  more  attracted,  and  move  through  a  greater  distance  than 
the  farther  ones.  When  the  nearest  molecules  strike  the  sphere, 
they  are  repelled,  and  collisions  occur  at  all  distances  within  the 
inductive  action  of  the  sphere.  It  is  now  clear  that,  if  the  poten- 


266  INVENTIONS  OF  NIKOLA   TE8LA. 

tial  be  steady,  but  little  loss  of  energy  can  be  caused  in  this  way, 
for  the  molecules  which  are  nearest  to  the  sphere,  having  had  an 
additional  charge  imparted  to  them  by  contact,  are  not  attracted 
until  they  have  parted,  if  not  with  all,  at  least  with  most  of  the 
additional  charge,  which  can  be  accomplished  only  after  a  great 
many  collisions.  From  the  fact,  that  with  a  steady  potential 
there  is  but  little  loss  in  dry  air,  one  must  come  to  such  a  con- 
clusion. When  the  potential  of  a  sphere,  instead  of  being  steady, 
is  alternating,  the  conditions  are  entirely  different.  In  this  case 
a  rhythmical  bombardment  occurs,  no  matter  whether  the  mole- 
cules, after  coming  in  contact  with  the  sphere,  lose  the  imparted 


FIG.  156. 

charge  or  not ;  what  is  more,  if  the  charge  is  not  lost,  the  impacts 
are  only  the  more  violent.  Still,  if  the  frequency  of  the  im- 
pulses be  very  small,  the  loss  caused  by  the  impacts  and  collisions 
would  not  be  serious,  unless  the  potential  \vere  excessive.  But 
when  extremely  high  frequencies  and  more  or  less  high  potentials 
are  used,  the  loss  may  very  great.  The  total  energy  lost  per  unit 
of  time  is  proportionate  to  the  product  of  the  number  of  impacts 
per  second,  or  the  frequency  and  the  energy  lost  in  each  impact. 
But  the  energy  of  an  impact  must  be  proportionate  to  the  square 
of  the  electric  density  of  the  sphere,  since  the  charge  imparted 


HIGH  FREQUENCY  AND  HIGH  POTENTIAL  CURRENTS.     267 

to  the  molecule  is  proportionate  to  that  density.  I  conclude  from 
this  that  the  total  energy  lost  must  be  proportionate  to  the  pro- 
duct of  the  frequency  and  the  square  of  the  electric  density ;  but 
this  law  needs  experimental  confirmation.  Assuming  the  pre- 
ceding- considerations  to  be  true,  then,  by  rapidly  alternating  the 
potential  of  a  body  immersed  in  an  insulating  gaseous  medium, 
any  amount  of  energy  may  be  dissipated  into  space.  Most  of 
that  energy  then,  I  believe,  is  not  dissipated  in  the  form  of  long 
ether  waves,  propagated  to  considerable  distance,  as  is  thought 
most  generally,  but  is  consumed — in  the  case  of  an  insulated 
sphere,  for  example — in  impact  and  collisional  losses — that  is, 
heat  vibrations — on  the  surface  and  in  the  vicinity  of  the  sphere. 
To  reduce  the  dissipation,  it  is  necessary  to  work  with  a  small 
electric  density — the  smaller,  the  higher  the  frequency. 

But  since,  on  the  assumption  before  made,  the  loss  is  dimin- 
ished with  the  square  of  the  density,  and  since  currents  of  very 
high  frequencies  involve  considerable  waste  when  transmitted 
through  conductors,  it  follows  that,  on  the  whole,  it  is  better  to 
employ  one  wire  than  two.  Therefore,  if  motors,  lamps,  or  de- 
vices of  any  kind  are  perfected,  capable  of  being  advantageously 
operated  by  currents  of  extremely  high  frequency,  economical 
reasons  will  make  it  advisable  to  use  only  one  wire,  especially  if 
the  distances  are  great. 

When  energy  is  absorbed  in  a  condenser,  the  same  behaves  as 
though  its  capacity  were  increased.  Absorption  always  exists 
more  or  less,  but  generally  it  is  small  and  of  no  consequence  us 
long  as  the  frequencies  are  not  very  great,  In  using  extremely 
high  frequencies,  and,  necessarily  in  such  case,  also  high  poten- 
tials, the  absorption — or,  what  is  here  meant  more  particularly 
by  this  term,  the  loss  of  energy  due  to  the  presence  of  "a  gaseous 
medium — is  an  important  factor  to  be  considered,  as  the  energy 
absorbed  in  the  air  condenser  may  be  any  fraction  of  the  supplied 
energy.  This  would  seem  to  make  it  very  difficult  to  tell  from 
the  measured  or  computed  capacity  of  an  air  condenser  its  actual 
capacity  or  vibration  period,  especially  if  the  condenser  is  of  very 
small  surface  and  is  charged  to  a  very  high  potential.  As  many 
important  results  are  dependent  upon  the  correctness  of  the 
estimation  of  the  vibration  period,  this  subject  demands  the  most 
careful  scrutiny  of  other  investigators.  To  reduce  the  probable 
error  as  much  as  possible  in  experiments  of  the  kind  alluded  to, 
it  is  advisable  to  use  spheres  or  plates  of  large  surface,  so  as  to 


268  INVENTIONS  OF  NIKOLA  TEFL  A. 

make  the  density  exceedingly  small.  Otherwise,  when  it  is 
practicable,  an  oil  condenser  should  be  used  in  preference.  In 
oil  or  other  liquid  dielectrics  there  are  seemingly  no  such  losses 
as  in  gaseous  media.  It  being  impossible  to  exclude  entirely  the 
gas  in  condensers  with  solid  dielectrics,  such  condensers  should 
be  immersed  in  oil,  for  economical  reasons,  if  nothing  else  ;  they 
can  then  be  strained  to  the  utmost,  and  will  remain  cool.  In 
Leyden  jars  the  loss  due  to  air  is  comparatively  small,  as  the  tin- 
foil coatings  are  large,  close  together,  and  the  charged  surfaces 
not  directly  exposed ;  but  when  the  potentials  are  very  high,  the 
loss  may  be  more  or  less  considerable  at,  or  near,  the  upper  edge 
of  the  foil,  where  the  air  is  principally  acted  upon.  If  the  jar 
be  immersed  in  boiled-out  oil,  it  will  be  capable  of  performing 
four  times  the  amount  of  work  which  it  can  for  any  length  of 
time  when  used  in  the  ordinary  way,  and  the  loss  will  be  inappre- 
ciable. 

It  should  not  be  thought  that  the  loss  in  heat  in  an  air  con- 
denser is  necessarily  associated  with  the  formation  of  /-/VJ/r 
streams  or  brushes.  If  a  small  electrode,  inclosed  in  an  un- 
exhausted bulb,  is  connected  to  one  of  the  terminals  of  the  coil, 
streams  can  be  seen  to  issue  from  the  electrode,  and  the  air  in  the 
bulb  is  heated ;  if  instead  of  a  small  electrode  a  large  sphere  is 
inclosed  in  the  bulb,  no  streams  are  observed,  still  the  air  is 
heated. 

j^or  should  it  be  thought  that  the  temperature  of  an  air  con- 
denser would  give  even  an  approximate  idea  of  the  loss  in  heat 
incurred,  as  in  such  case  heat  must  be  given  off  much  more 
quickly,  since  there  is,  in  addition  to  the  ordinary  radiation,  a 
very  active  carrying  away  of  heat  by  independent  carriers  going 
on,  and  since  not  only  the  apparatus,  but  the  air  at  some  distance 
from  it  is  heated  in  consequence  of  the  collisions  which  must 
occur. 

Owing  to  this,  in  experiments  with  such  a  coil,  a  rise  of  tem- 
perature can  be  distinctly  observed  only  when  the  body  connected 
to  the  coil  is  very  small.  But  with  apparatus  on  a  larger  scale, 
even  a  body  of  considerable  bulk  would  be  heated,  as,  for  instance, 
the  body  of  a  person ;  and  I  think  that  skilled  physicians  might 
make  observations  of  utility  in  such  experiments,  which,  if  the 
apparatus  were  judiciously  designed,  would  not  present  the  slight- 
est danger. 

A  question  of   some  interest,  principally   to  meteorologists, 


HIGH  FEEQUENCY  AND  HIGH  POTENTIAL  CURRENTS.     269 

presents  itself  here.  How  does  the  earth  behave  ?  The  earth  is 
an  air  condenser,  but  is  it  a  perfect  or  a  very  imperfect  one — a 
mere  sink  of  energy  ?  There  can  be  little  doubt  that  to  such 
small  disturbance  as  might  be  caused  in  an  experiment,  the  earth 
behaves  as  an  almost  perfect  condenser.  But  it  might  be  differ- 
ent when  its  charge  is  set  in  yibration  by  some  sudden  disturb- 
ance occurring  in  the  heavens.  In  such  case,  as  before  stated, 
probably  only  little  of  the  energy  of  the  vibrations  set  up  would 
be  lost  into  space  in  the  form  of  long  ether  radiations,  but  most 
of  the  energy,  I  think,  would  spend  itself  in  molecular  impacts 
and  collisions,  and  pass  off  into  space  in  the  form  of  short  heat, 
and  possibly  light,  waves.  As  both  the  frequency  of  the  vibra- 
tions of  the  charge  and  the  potential  are  in  all  probability  exces- 
sive, the  energy  converted  into  heat  may  be  considerable.  Since 
the  density  must  be  unevenly  distributed,  either  in  consequence 
of  the  irregularity  of  the  earth's  surface,  or  on  account  of  the 
condition  of  the  atmosphere  in  various  places,  the  effect  produced 
would  accordingly  vary  from  place  to  place.  Considerable  varia- 
tions in  the  temperature  and  pressure  of  the  atmosphere  may  in 
this  manner  be  caused  at  any  point  of  the  surface  of  the  earth. 
The  variations  may  be  gradual  or  very  sudden,  according  to  the 
nature  of  the  general  disturbance,  and  may  produce  rain  and 
storms,  or  locally  modify  the  weather  in  any  way. 

From  the  remarks  before  made,  one  may  see  what  an  import- 
ant factor  of  loss  the  air  in  the  neighborhood  of  a  charged  surface 
becomes  when  the  electric  density  is  great  and  the  frequency  of 
the  impulses  excessive.  But  the  action,  as  explained,  implies 
that  the  air  is  insulating — that  is,  that  it  is  composed  of  independ- 
ent carriers  immersed  in  an  insulating  medium.  This  is  the  case 
only  when  the  air  is  at  something  like  ordinary  or  greater,  or  at 
extremely  small,  pressure.  When  the  air  is  slightly  rareiied  and 
conducting,  then  true  conduction  losses  occur  also.  In  such  case, 
of  course,  considerable  energy  may  be  dissipated  into  space  even 
with  a  steady  potential,  or  with  impulses  of  low  frequency,  if  the 
density  is  very  great. 

When  the  gas  is  -at  very  low  pressure,  an  electrode  is  heated 
more  because  higher  speeds  can  be  reached.  If  the  gas  around 
the  electrode  is  strongly  compressed,  the  displacements,  and 
consequently  the  speeds,  are  very  small,  and  the  heating  is  in- 
signincant.  But  if  in  such  case  the  frequency  could  be  suffici- 
ently increased,  the  electrode  would  be  brought  to  a  high  tern- 


270 


INVENTIONS  OF  NIKOLA  TESLA. 


perature  as  well  as  if  the  gas  were  at  very  low  pressure  ;  in  fact, 
exhausting  the  bulb  is  only  necessary  because  we  cannot  produce, 
(and  possibly  not  convey)  currents  of  the  required  frequency. 

Returning  to  the  subject  of  electrode  lamps,  it  is  obviously  of 
advantage  in  such  a  lamp  to  confine  as  much  as  possible  the  heat 
to  the  electrode  by  preventing  the  circulation  of  the  gas  in  the 
bulb.  If  a  very  small  bulb  be  taken,  it  would  confine  the  heat 
better  than  a  large  one,  but  it  might  not  be  of  sufficient  capacity 
to  be  operated  from  the  coil,  or,  if  so,  the  glass  might  get  too 
hot.  A  simple  way  to  improve  in  this  direction  is  to  employ  a 
globe  of  the  required  size,  but  to  place  a  small  bulb,  the  diameter 
of  which  is  properly  estimated,  over  the  refractory  button  con- 


FIG.  157. 

tained  in  the  globe.  This  arrangement  is  illustrated  in  Fig.  157. 
The  globe  L  has  in  this  case  a  large  neck  n,  allowing  the  small 
bulb  b  to  slip  through.  Otherwise  the  construction  is  the  same 
as  shown  in  Fig.  147,  for  example.  The  small  bulb  is  conveni- 
ently supported  upon  the  stem  s,  carrying  the  refractory  button 
in.  It  is  separated  from  the  aluminum  tube  a  by  several  layers 
of  mica  M,  in  order  to  prevent  the  cracking  of  the  neck  by  the 
rapid  heating  of  the  aluminum  tube  upon  a  sudden  turning  on 
of  the  current.  The  inside  bulb  should  be  as  small  as  possible 
when  it  is  desired  to  obtain  light  only  by  incandescence  of  the 
electrode.  If  it  is  desired  to  produce  phosphorescence,  the  bulb 


man  FEEQUENCT  AND  HIGH  POTENTIAL  CUERENTS.    271 

should  be  larger,  else  it  would  be  apt  to  get  too  hot,  and  the 
phosphorescence  would  cease.  In  this  arrangement  usually  only 
the  small  bulb  shows  phosphorescence,  as  there  is  practically  no 
bombardment  against  the  outer  globe.  In  some  of  these  bulbs 
constructed  as  illustrated  in  Fig.  157,  the  small  tube  was  coated 
with  phosphorescent  paint,  and  beautiful  effects  were  obtained. 
Instead  of  making  the  inside  bulb  large,  in  order  to  avoid  undue 
heating,  it  answers  the  purpose  to  make  the  electrode  m  larger. 
In  this  case  the  bombardment  is  weakened  by  reason  of  the 
smaller  electric  density. 

Many  bulbs  were  constructed  on  the  plan  illustrated  in  Fig. 
158.  Here  a  small  bulb  Z>,  containing  the  refractory  button  >//, 
upon  being  exhausted  to  a  very  high  degree  Avas  sealed  in  a  large 
globe  L,  which  wTas  then  moderately  exhausted  and  sealed  off. 
The  principal  advantage  of  this  construction  was  that  it  allowed 
of  reaching  extremely  high  vacua,  and,  at  the  same  time  of  using  a 
large  bulb.  It  was  found,  in  the  course  of  experiments  with 
bulbs  such  as  illustrated  in  Fig.  158,  that  it  was  well  to  make 
the  stem  *,  near  the  seal  at  <*,  very  thick,  and  the  leading-in  wire 
//•  thin,  as  it  occurred  sometimes  that  the  stem  at  e  was  heated 
and  the  bulb  was  cracked.  Often  the  outer  globe  L  was  exhausted 
only  just  enough  to  allow  the  discharge  to  pass  through,  and  the 
space  between  the  bulbs  appeared  crimson,  producing  a  curious 
effect.  In  some  cases,  when  the  exhaustion  in  globe  L  was  very 
low,  and  the  air  good  conducting,  it  was  found  necessary,  in  order 
to  bring  the  button  in  to  high  incandescence,  to  place,  preferably 
on  the  upper  part  of  the  neck  of  the  globe,  a  tinfoil  coating  which 
was  connected  to  an  insulated  body,  to  the  ground,  or  to  the 
other  terminal  of  the  coil,  as  the  highly  conducting  air  weakened 
the  effect  somewhat,  probably  by  being  acted  upon  inductively 
from  the  wire  w,  where  it  entered  the  bulb  at  e.  Another  diffi- 
culty— which,  however,  is  always  present  when  the  refractory 
button  is  mounted  in  a  very  small  bulb — existed  in  the  construc- 
tion illustrated  in  Fig.  158,  namely,  the  vacuum  in,  the  bulb  1> 
would  be  impaired  in  a  comparatively  short  time. 

The  chief  idea  in  the  two  last  described  constructions  was  to 
confine  the  heat  to  the  central  portion  of  the  globe  by  preventing 
the  exchange  of  air.  An  advantage  is  secured,  but  owing  to  the 
heating  of  the  inside  bulb  and  slow  evaporation  of  the  glass,  the 
vacuum  is  hard  to  maintain,  even  if  the  construction  illustrated 
in  Fig.  157  be  chosen,  in  which  both  bulbs  communicate. 


272  INVENTIONS  OF  NIKOLA  TE8LA, 

But  by  far  the  better  way — the  ideal  way — would  be  to  reach 
sufficiently  high  frequencies.  The  higher  the  frequency,  the 
slower  would  be  the  exchange  of  the  air,  and  I  think  that  a  fre- 
quency may  be  reached,  at  which  there  would  be  no  exchange 
whatever  of  the  air  molecules  around  the  terminal.  We  would 
then  produce  a  flame  in  which  there  would  be  no  carrying  away 
of  material,  and  a  queer  flame  it  would  be,  for  it  would  be  rigid ! 
With  such  high  frequencies  the  inertia  of  the  particles  would  come 
into  play.  As  the  brush,  or  flame,  would  gain  rigidity  in  virtue 
of  the  inertia  of  the  particles,  the  exchange  of  the  latter  would 
be  prevented.  This  would  necessarily  occur,  for,  the  number  of 
impulses  being  augmented,  the  potential  energy  of  each  would 
diminish,  so  that  finally  only  atomic  vibrations  could  be  set  up, 
and  the  motion  of  translation  through  measurable  space  would 
cease.  Thus  an  ordinary  gas  burner  connected  to  a  source  of 
rapidly  alternating  potential  might  have  its  efficiency  augmented 
to  a  certain  limit,  and  this  for  two  reasons — because  of  the  addi- 
tional vibration  imparted,  and  because  of  a  slowing  down  of  the 
process  of  carrying  off.  But  the  renewal  being  rendered  difficult, 
a  renewal  being  necessary  to  maintain  the  burner,  a  continued 
increase  of  the  frequency  of  the  impulses,  assuming  they  could 
be  transmitted  to  and  impressed  upon  the  flame,  would  result  in 
the  "  extinction "  of  the  latter,  meaning  by  this  term  only  the 
cessation  of  the  chemical  process. 

I  think,  however,  that  in  the  case  of  an  electrode  immersed  in 
a  fluid  insulating  medium,  and  surrounded  by  independent  car- 
riers of  electric  charges,  which  can  be  acted  upon  inductively,  a 
sufficient  high  frequency  of  the  impulses  would  probably  result 
in  a  gravitation  of  the  gas  all  around  toward  the  electrode.  For 
this  it  would  be  only  necessary  to  assume  that  the  independent 
bodies  are  irregularly  shaped ;  they  would  then  turn  toward  the 
electrode  their  side  of  the  greatest  electric  density,  and  this 
would  be  a  position  in  which  the  fluid  resistance  to  approach 
.would  be  smaller  than  that  offered  to  the  receding. 

The  general  opinion,  I  do  not  doubt,  is  that  it  is  out  of  the 
question  to  reach  any  such  frequencies  as  might — assuming  some 
of  the  views  before  expressed  to  be  true — produce  any  of  the  re. 
suits  which  I  have  pointed  out  as  mere  possibilities.  This  may  be 
so,  but  in  the  course  of  these  investigations,  from  the  observation 
of  many  phenomena,  I  have  gained  the  conviction  that  these  fre- 
quencies would  be  much  lower  than  one  is  apt  to  estimate  at 


HIGH  FREQUENCY  AND  HIGH  POTENTIAL  CURRENTS.     273 

first.  In  a  flame  we  set  up  light  vibrations  by  causing  molecules, 
or  atoms,  to  collide.  But  what  is  the  ratio  of  the  frequency  of 
the  collisions  and  that  of  the  vibrations  set  up?  Certainly  it 
must  be  incomparably  smaller  than  that  of  the  strokes  of  the  bell 
and  the  sound  vibrations,  or  that  of  the  discharges  and  the  oscil- 
lations of  the  condenser.  We  may  cause  the  molecules  of  the 
gas  to  collide  by  the  use  of  alternate  electric  impulses  of  high 
frequency,  and  so  we  may  imitate  the  process  in  a  flame ;  and 
from  experiments  with  frequencies  which  we  are  now  able  to 
obtain,  I  think  that  the  result  is  producible  with  impulses  which 
are  transmissible  through  a  conductor. 

In  connection  with  thoughts  of  a  similar  nature,  it  appeared  to 
me  of  great  interest  to  demonstrate  the  rigidity  of  a  vibrating  gas- 
eous column.  Although  with  such  low  frequencies  as,  say  10,000 
per  second,  which  I  was  able  to  obtain  without  difficulty  from  a 
specially  constructed  alternator,  the  task  looked  discouraging  at 
first,  I  made  a  series  of  experiments.  The  trials  with  air  at  ordi- 
nary pressure  led  to  no  result,  but  with  air  moderately  rarefied  I 
obtain  what  I  think  to  be  an  unmistakable  experimental  evidence 
of  the  property  sought  for.  As  a  result  of  this  kind  might  lead 
able  investigators  to  conclusions  of  importance,  I  will  describe 
one  of  the  experiments  performed. 

It  is  well  known  that  when  a  tube  is  slightly  exhausted,  the 
discharge  may  be  passed  through  it  in  the  form  of  a  thin  lumin- 
ous thread.  When  produced  with  currents  of  low  frequency, 
obtained  from  a  coil  operated  as  usual,  this  thread  is  inert.  If  a 
magnet  be  approached  to  it,  the  part  near  the  same  is  attracted 
or  repelled,  according  to  the  direction  of  the  lines  of  force  of  the 
magnet.  It  occurred  to  me  that  if  such  a  thread  would  be  pro- 
duced with  currents  of  very  high  frequency,  it  should  be  more 
or  less  rigid,  and  as  it  was  visible  it  could  be  easily  studied. 
Accordingly  I  prepared  a  tube  about  one  inch  in  diameter  and 
one  metre  long,  with  outside  coating  at  each  end.  The  tube  was 
exhausted  to  a  point  at  which,  by  a  little  working,  the  thread  dis- 
charge could  be  obtained.  It  must  be  remarked  here  that  the 
general  aspect  of  the  tube,  and  the  degree  of  exhaustion,  are 
quite  other  than  when  ordinary  low  frequency  currents  are 
used.  As  it  was  found  preferable  to  work  with  one  terminal, 
the  tube  prepared  was  suspended  from  the  end  of  a  wire  con- 
nected to  the  terminal,  the  tinfoil  coating  being  connected  to  the 
wire,  and  to  the  lower  coating  sometimes  a  small  insulated  plate 


274  INVENTIONS  OF  NIKOLA  TESLA. 

was  attached.  When  the  thread  was  formed,  it  extended  through 
the  upper  part  of  the  tube  and  lost  itself  in  the  lower  end.  If  it 
possessed  rigidity  it  resembled,  not  exactly  an  elastic  cord 
stretched  tight  between  two  supports,  but  a  cord  suspended  from 
a  height  with  a  small  weight  attached  at  the  end.  When  the 
finger  or  a  small  magnet  was  approached  to  the  upper  end  of  the 
luminous  thread,  it  could  be  brought  locally  out  of  position  by 
electrostatic  or  magnetic  action  ;  and  when  the  disturbing  object 
was  very  quickly  removed,  an  analogous  result  was  produced,  as 
though  a  suspended  cord  would  be  displaced  and  quickly  released 
near  the  point  of  suspension.  In  doing  this  the  luminous  thread 
was  set  in  vibration,  and  two  very  sharply  marked  nodes,  and  a 
third  indistinct  one,  were  formed.  The  vibration,  once  set  up, 
continued  for  fully  eight  minutes,  dying  gradually  out.  The 
speed  of  the  vibration  often  varied  perceptibly,  and  it  could  be 
observed  that  the  electrostatic  attraction  of  the  glass  affected  the 
vibrating  thread ;  but  it  was  clear  that  the  electrostatic  action 
was  not  the  cause  of  the  vibration,  for  the  thread  was  most  gen- 
erally stationary,  and  could  always  be  set  in  vibration  by  passing 
the  finger  quickly  near  the  upper  part  of  the  tube.  With  a 
magnet  the  thread  could  be  split  in  two  and  both  parts  vibrated. 
By  approaching  the  hand  to  the  lower  coating  of  the  tube,  or 
insulation  plate  if  attached,  the  vibration  was  quickened  ;  also,  as 
far  as  I  could  see,  by  raising  the  potential  or  frequency.  Thus, 
either  increasing  the  frequency  or  passing  a  stronger  discharge 
of  the  same  frequency  corresponded  to  a  tightening  of  the  cord. 
I  did  not  obtain  any  experimental  evidence  with  condenser  dis- 
charges. A  luminous  band  excited  in  the  bulb  by  repeated  dis- 
charges of  a  Leyden  jar  must  possess  rigidity,  and  if  deformed 
and  suddenly  released,  should  vibrate.  But  probably  the  amount 
of  vibrating  matter  is  so  small  that  in  spite  of  the  extreme  speed, 
the  inertia  cannot  prominently  assert  itself.  Besides,  the  obser- 
vation in  such  a  case  is  rendered  extremely  difficult  on  account 
of  the  fundamental  vibration. 

The  demonstration  of  the  fact — which  still  needs  better  ex- 
perimental confirmation — that  a  vibrating  gaseous  column  pos- 
sesses rigidity,  might  greatly  modify  the  views  of  thinkers. 
When  with  low  frequencies  and  insignificant  potentials  indications 
of  that  property  may  be  noted,  how  must  a  gaseous  medium  be- 
liave  under  the  influence  of  enormous  electrostatic  stresses  which 
may  be  active  in  the  interstellar  space,  and  which  may  alternate 


HTGH  FREQUKNCY  AND  HIGH  POTENTIAL  CURRENTS.     275 

with  inconceivable  rapidity  ?  The  existence  of  such  an  electro- 
static, rhythmically  throbbing  force — of  a  vibrating  electrostatic 
field — would  show  a  possible  way  how  solids  might  have  formed 
from  the  ultra-gaseous  uterus,  and  how  transverse  and  all  kinds 
of  vibrations  may  be  transmitted  through  a  gaseous  medium  fill- 
ing all  space.  Then,  ether  might  be  a  true  fluid,  devoid  of 
rigidity,  and  at  rest,  it  being  merely  necessary  as  a  connecting 
link  to  enable  interaction.  What  determines  the  rigidity  of  a 
body  ?  It  must  be  the  speed  and  the  amount  of  motive  matter. 
In  a  gas  the  speed  may  be  considerable,  but  the  density  is  exceed- 
ingly small ;  in  a  liquid  the  speed  would  be  likely  to  be  small, 
though  the  density  may  be  considerable ;  and  in  both  cases  the 
inertia  resistance  offered  to  displacement  is  practically  nil.  But 
place  a  gaseous  (or  liquid)  column  in  an  intense,rapidly  alternating 
electrostatic  field,  set  the  particles  vibrating  with  enormous 
speeds,  then  the  inertia  resistance  asserts  itself.  A  body  might 
move  with  more  or  less  freedom  through  the  vibrating  mass,  but 
as  a  whole  it  would  be  rigid. 

There  is  a  subject  which  I  must  mention  in  connection  with 
these  experiments :  it  is  that  of  high  vacua.  This  is  a  subject, 
the  study  of  which  is  not  only  interesting,  but  useful,  for  it  may 
lead  to  results  of  great  practical  importance.  In  commercial  ap-' 
paratus,  such  as  incandescent  lamps,  operated  from  ordinary 
systems  of  distribution,  a  much  higher  vacuum  than  is  obtained  at 
present  would  not  secure  a  very  great  advantage.  In  such  a  case 
the  work  is  performed  on  the  filament,  and  the  gas  is  little  con- 
cerned ;  the  improvement,  therefore,  would  be  but  trifling.  But 
when  we  begin  to  use  very  high  frequencies  and  potentials,  the 
action  of  the  gas  becomes  all  important,  and  the  degree  of  ex- 
haustion materially  modifies  the  results.  As  long  as  ordinary 
coils,  even  very  large  ones,  were  used,  the  study  of  the  subject 
was  limited,  because  just  at  a  point  when  it  became  most  inter- 
esting it  had  to  be  interrupted  on  account  of  the  "  non-striking  " 
vacuum  being  reached.  But  at  present  we  are  able  to  obtain 
from  a  small  disruptive  discharge  coil  potentials  much  higher 
than  even  the  largest  coil  was  capable  of  giving,  and,  what  is 
more,  we  can  make  the  potential  alternate  with  great  rapidity. 
Both  of  these  results  enable  us  now  to  pass  a  luminous  discharge 
through  almost  any  vacua  obtainable,  and  the  field  of  our  inves- 
tigations is  greatly  extended.  Think  we  as  we  may,  of  all  the 
possible  directions  to  develop  a  practical  illnminant,  the  line  of 


276  INVENTIONS  OF  NIKOLA  TESLA. 

high  vacua  seems  to  be  the  most  promising  at  present.  But  to 
reach  extreme  vacua  the  appliances  must  be  much  more  improved, 
and  ultimate  perfection  will  not  be  attained  until  we  shall  have 
discharged  the  mechanical  and  perfected  an  electrical  vacuum 
pump.  Molecules  and  atoms  can  be  thrown  out  of  a  bulb  under 
the  action  of  an  enormous  potential :  this  will  be  the  principle 
of  the  vacuum  pump  of  the  future.  For  the  present,  we  must 
secure  the  best  results  we  can  with  mechanical  appliances.  In 
this  respect,  it  might  not  be  out  of  the  way  to  say  a  few  words 
about  the  method  of,  and  apparatus  for,  producing  excessively 


FIG.  159. 

high  degrees  of  exhaustion  of  which  I  have  availed  myself  in  the 
course  of  these  investigations.  It  is  very  probable  that  other  ex- 
perimenters have  used  similar  arrangements ;  but  as  it  is  possible 
that  there  may  be  an  item  of  interest  in  their  description,  a  few 
remarks,  which  will  render  this  investigation  more  complete, 
might  be  permitted. 

The  apparatus  is  illustrated  in  a  drawing  shown  in  Fig.  159. 
s  represents  a  Sprengel  pump,  which  has  been  specially  con- 
structed to  better  suit  the  work  required.  The  stop-cock  which 


HIGH  FREQUENCY  AND  HIGH  POTENTIAL  CURRENTS.     277 

is  usually  employed  has  been  omitted,  and  instead  of  it  a  hollow 
stopper  s  has  been  fitted  in  the  neck  of  the  reservoir  K.  This 
stopper  has  a  small  hole  A,  through  which  the  mercury  descends  ; 
the  size  of  the  outlet  o  being  properly  determined  with  respect 
to  the  section  of  the  fall  tube  £,  which  is  sealed  to  the  reservoir 
instead  of  being  connected  to  it  in  the  usual  manner.  This 
arrangement  overcomes  the  imperfections  and  troubles  which 
often  arise  from  the  use  of  the  stopcock  on  the  reservoir  and  the 
connections  of  the  latter  with  the  fall  tube. 

The  pump  is  connected  through  a  (J-snaped  tube  t  to  a  very 
large  reservoir  &lm  Especial  care  was  taken  in  fitting  the  grind- 
ing surfaces  of  the  stoppers  p  and  plt  and  both  of  these  and  the 
mercury  caps  above  them  were  made  exceptionally  long.  After 
the  U-shaped  tube  was  fitted  and  put  in  place,  it  was  heated,  so 
as  to  soften  and  take  off  the  strain  resulting  from  imperfect 
fitting.  The  (J -shaped  tube  was  provided  with  a  stopcock  c. 
and  two  ground  connections  y  and  yl — one  for  a  small  bulb  b, 
usually  containing  caustic  potash,  and  the  other  for  the  receiver 
/•,  to  be  exhausted. 

The  reservoir  Eb  was  connected  by  means  of  a  rubber  tube  to 
a  slightly  larger  reservoir  R^,  each  of  the  two  reservoirs  being 
provided  with  a  stopcock  ct  and  c2,  respectively.  The  reservoir 
RJ  could  be  raised  and  lowered  by  a  wheel  and  rack,  and  the 
range  of  its  motion  was  so  determined  that  when  it  was  filled 
with  mercury  and  the  stopcock  c2  closed,  so  as  to  form  a  Torri- 
cellian vacuum  in  it  when  raised,  it  could  be  lifted  so  high  that 
the  reservoir  EJ  would  stand  a  little  above  stopcock  c^  ;  and  when 
this  stopcock  was  closed  and  the  reservoir  Eg  descended,  so  as  to 
form  a  Torricellian  vacuum  in  reservoir  R,,  it  could  be  lowered 
so  far  as  to  completely  empty  the  latter,  the  mercury  filling  the 
reservoir  RJ  up  to  a  little  above  stopcock  c2. 

The  capacity  of  the  pump  and  of  the  connections  was  taken 
as  small  as  possible  relatively  to  the  volume  of  reservoir,  El5 
since,  of  course,  the  degree  of  exhaustion  depended  upon  the 
ratio  of  these  quantities. 

With  this  apparatus  I  combined  the  usual  means  indicated  by 
former  experiments  for  the  production  of  very  high  vacua.  In 
most  of  the  experiments  it  was  most  convenient  to  use  caustic 
potash.  I  may  venture  to  say,  in  regard  to  its  use,  that  much 
time  is  saved  and  a  more  perfect  action  of  the  pump  insured  by 
fusing  and  boiling  the  potash  as  soon  as,  or  even  before,  the 


278  INVENTIONS  OF  NIKOLA   TESLA. 

pump  settles  down.  If  this  course  is  not  followed,  the  sticks,  as 
ordinarily  employed,  may  give  off  moisture  at  a  certain  very 
slow  rate,  and  the  pump  may  work  for  many  hours  without 
reaching  a  very  high  vacuum.  The  potash  was  heated  either  hy 
a  spirit  lamp  or  by  passing  a  discharge  through  it,  or  by  passing 
a  current  through  a  wire  contained  in  it.  The  advantage  in  the 
latter  case  was  that  the  heating  couldjbe  more  rapidly  repeated. 
Generally  the  process  of  exhaustion  was  the  following : — At 
the  start,  the  stop-cocks  c  and  ct  being  open,  and  all  other  con- 
nections closed,  the  reservoir  n>  was  raised  so  far  that  the  mer- 
cury filled  the  reservoir  Rt  and  a  part  of  the  narrow  connecting 
U-shaped  tube.  When  the  pump^was  set  to  work,  the  mercury 
would,  of  course,  quickly  rise  in  the  tube,  and  reservoir  RJ  was 
lowered,  the  experimenter  keeping  ^the  mercury  at  about  the 
same  level.  The  reservoir  RJ  wasj  balanced  by  a  long  spring 
which  facilitated  the  operation,  and  the  friction  of  the  parts  was 
generally  sufficient  to  keep  it  in  almost  any  position.  When  the 
Sprengel  pump  had  done  its  work,  the'reservoir  R%  was  further  low- 
ered and  the  mercury  descended  in  Rt  and  tilled  R>,  whereupon  stop- 
cock 02  was  closed.  The  air  adhering  to  the  walls  of  Rt  and  that 
absorbed  by  the  mercury  was  carried  off,  and  to  free  the  mercury 
of  all  air  the  reservoir  E2  was  for  a  long  time  worked  up  and 
down.  During  this  process  some  air,  which  would  gather  below 
stopcock  c2,  was  expelled  from  R2  by  lowering  it  far  enough  and 
opening  the  stopcock,  closing  the  latter  again  before  raising  the 
reservoir.  When  all  the  air  had  been  expelled  from  the  mercury, 
and  no  air  would  gather  in  Rg  when  it  was  lowered,  the  caustic 
potash  was  resorted  to.  The  reservoir  RJ  was  now  again  raised 
until  the  mercury  in  RJ  stood  above  stopcock  Ci.  The  caustic 
potash  was  fused  and  boiled,  and  moisture 'partly  carried  off  by 
the  pump  and  partly  re-absorbed ;  and  this  process  of  heating 
and  cooling  was  repeated  many  times,  and  each  time,  upon  the 
moisture  being  absorbed  or  carried  off,  the  reservoir  B^  was  for 
a  long  time  raised  and  lowered.  In  this  manner  all  the  moisture 
was  carried  off  from  the  mercury,  and  both  the  reservoirs  were 
in  proper  condition  to  be  used.  The  reservoir  R2  was  then  again 
raised  to  the  top,  and  the  pump  was  kept  working  for  a  long 
time.  When  the  highest  vacuum  obtainable  with  the  pump  had 
been  reached,  the  potash  bulb  was  usually  wrapped  with  cotton 
which  was  sprinkled  with  ether  so  as  to  keep  the  potash  at  a 
very  low  temperature,  then  the  reservoir  R2  was  lowered,  and  upon 
reservoir  Rt  being  emptied  the  receiver  was]quickly  sealed  up. 


HIGH  FREQUENCY  AND  HIGH  POTENTIAL  CURRENTS.      279 

When  a  new  bulb  was  put  on,  the  mercury  was  always  raised 
above  stopcock  c,,  which  was  closed,  so  as  to  always  keep  the 
mercury  and  both  the  reservoirs  in  fine  condition,  and  the  mer- 
cury was  never  withdrawn  from  Rt  except  when  the  pump  had 
reached  the  highest  degree  of  exhaustion.  It  is  necessary  to  ob- 
serve this  rule  if  it  is  desired  to  use  the  apparatus  to  advantage. 

By  means  of  this  arrangement  I  was  able  to  proceed  very 
quickly,  and  when  the  apparatus  was  in  perfect  order  it  was  pos- 
sible to  reach  the  phosphorescent  stage  in  a  small  bulb  in  less 
than  fifteen  minutes,  which  is  certainly  very  quick  work  for  a 
small  laboratory  arrangement  requiring  all  in  all  about  100  pounds 
of  mercury.  With  ordinary  small  bulbs  the  ratio  of  the  capacity 
of  the  pump,  receiver,  and  connections,  and  that  of  reservoir  R 
was  about  1  to  20,  and  the  degrees  of  exhaustion  reached  were 
necessarily  very  high,  though  I  am  unable  to  make  a  precise  and 
reliable  statement  how  far  the  exhaustion  was  carried. 

What  impresses  the  investigator  most  in  the  course  of  these 
experiences  is  the  behavior  of  gases  when  subjected  to  great^rap- 
idly  alternating^  electrostatic  stresses.  But  he  must  remain  in 
doubt  as  to  whether  the  effects  observed  are  due  wholly  to  the 
molecules,  or  atoms,  of  the  gas  which  chemical  analysis  discloses 
to  us,  or  whether  there  enters  into  play  another  medium  of  a 
gaseous  nature,  comprising  atoms,  or  molecules,  immersed  in  a 
fluid  pervading  the  space.  Such  a  medium  surely  must  exist, 
and  I  am  convinced  that,  for  instance,  even  if  air  were  absent, 
the  surface  and  neighborhood  of  a  body  in  space  would  be  heated 
by  rapidly  alternating  the  potential  of  the  body;  but  no  such 
heating  of  the  surface  or  neighborhood  could  occur  if  all  free 
atoms  were  removed  and  only  a  homogeneous,  incompressible,  and 
elastic  fluid — such  as  ether  is  supposed  to  be — would  remain,  for 
then  there  would  be  no  impacts,  no  collisions.  In  such  a  case, 
as  far  as  the  body  itself  is  concerned,  only  f rictional  losses  in  the 
inside  could  occur. 

It  is  a  striking  fact  that  the  discharge  through  a  gas  is  es- 
tablished with  ever-increasing  freedom  as  the  frequency  of  the 
impulses  is  augmented.  It  behaves  in  this  respect  quite  contrarily 
to  a  metallic  conductor.  In  the  latter  the  impedance  enters 
prominently  into  play  as  the  frequency  is  increased,  but  the  gas 
acts  much  as  a  series  of  condensers  would  ;  the  facility  with 
which  the  discharge  passes  through,  seems  to  depend  on  the  rate 
of  change  of  potential.  If  it  acts  so,  then  in  a  vacuum  tube  even 


280  INVENTIONS  OF  NIKOLA  TESLA. 

of  great  length,  and  no  matter  how  strong  the  current,  self-in- 
duction could  not  assert  itself  to  any  appreciable  degree.  We 
have,  then,  as  far  as  we  can  now  see,  in  the  gas  a  conductor 
which  is  capable  of  transmitting  electric  impulses  of  any  fre- 
quency which  we  may  be  able  to  produce.  Could  the  frequency  be 
brought  high  enough,  then  a  queer  system  of  electric  distribution, 
which  would  be  likely  to  interest  gas  companies,  might  be  real- 
ized :  metal  pipes  filled  with  gas — the  metal  being  the  insulator, 
the  gas  the  conductor — supplying  phosphorescent  bulbs,  or  per- 
haps devices  as  yet  uninvented.  It  is  certainly  possible  to  take 
a  hollow  core  of  copper,  rarefy  the  gas  in  the  same,  and  by  pas- 
sing impulses  of  sufficiently  high  frequency  through  a  circuit 
around  it,  bring  the  gas  inside  to  a  high  degree  of  incandescence  ; 
but  as  to  the  nature  of  the  forces  there  would  be  considerable 
uncertainty,  for  it  would  be  doubtful  whether  with  such  impulses 
the  copper  core  would  act  as  a  static  screen.  Such  paradoxes  and 
apparent  impossibilities  we  encounter  at  every  step  in  this  line  of 
work,  and  therein  lies,  to  a  great  extent,  the  charm  of  the  study. 
I  have  here  a  short  and  wide  tube  which  is  exhausted  to  a 
high  degree  and  covered  with  a  substantial  coating  of  bronze,  the 
coating  barely  allowing  the  light  to  shine  through.  A  metallic 
cap,  with  a  hook  for  suspending  the  tube,  is  fastened  around  the 
middle  portion  of  the  latter,  the  clasp  being  in  contact  with  the 
bronze  coating.  I  now  want  to  light  the  gas  inside  by  suspend- 
ing the  tube  on  a  wire  connected  to  the  coil.  Any  one  who 
would  try  the  experiment  for  the  first  time,  not  having  any  pre- 
vious experience,  would  probably  take  care  to  be  quite  alone 
when  making  the  trial,  for  fear  that  he  might  become  the  joke  of 
his  assistants.  Still,  the  bulb  lights  in  spite  of  the  metal  coating, 
and  the  light  can  be  distinctly  perceived  through  the  latter.  A 
long  tube  covered  with  aluminum  bronze  lights  when  held  in 
one  hand — the  other  touching  the  terminal  of  the  coil — quite 
powerfully.  It  might  be  objected  that  the  coatings  are  not 
sufficiently  conducting  ;  still,  even  if  they  were  highly  resistant, 
they  ought  to  screen  the  gas.  They  certainly  screen  it  perfectly 
in  a  condition  of  rest,  but  far  from  perfectly  when  the  charge 
is  surging  in  the  coating.  But  the  loss  of  energy  which  occurs 
within  the  tube,  notwithstanding  the  screen,  is  occasioned  prin- 
cipally by  the  presence  of  the  gas.  Were  we  to  take  a  large 
hollow  metallic  sphere  and  fill  it  with  a  perfect,  incompressible, 
fluid  dielectric,  there  would  be  no  loss  inside  of  the  sphere,  and 


HIGH  FREQUENCY  AND  HIGH  POTENTIAL  CURRENTS.     281 

consequently  the  inside  might  be  considered  as  perfectly  screened, 
though  the  potential  be  very  rapidly  alternating.  Even  were 
the  sphere  filled  with  oil,  the  loss  would  be  incomparably  smaller 
than  when  the  fluid  is  replaced  by  a  gas,  for  in  the  latter  case  the 
force  produces  displacements ;  that  means  impact  and  collisions 
in  the  inside. 

No  matter  what  the  pressure  of  the  gas  may  be,  it  becomes  an 
important  factor  in  the  heating  of  a  conductor  when  the  electric 
density  is  great  and  the  frequency  very  high.  That  in  the  heat- 
ing of  conductors  by  lightning  discharges,  air  is  an  element  of 
great  importance,  is  almost  as  certain  as  an  experimental  fact.  I 
may  illustrate  the  action  of  the  air  by  the  following  experiment: 
I  take  a  short  tube  which  is  exhausted  to  a  moderate  degree  and 
has  a  platinum  wire  running  through  the  middle  from  one  end 
to  the  other.  I  pass  a  steady  or  low  frequency  current  through 
the  wire,  and  it  is  heated  uniformly  in  all  parts.  The  heating 
here  is  due  to  conduction,  or  frictional  losses,  and  the  gas  around 
the  wire  has — as  far  as  we  can  see — no  function  to  perform. 
But  now  let  me  pass  sudden  discharges,  or  high  frequency  cur- 
rents, through  the  wire.  Again  the  wire  is  heated,  this  time 
principally  on  the  ends  and  least  in  the  middle  portion  ;  and  if 
the  frequency  of  the  impulses,  or  the  rate  of  change,  is  high 
enough,  the  wire  might  as  well  be  cut  in  the  middle  as  not,  for 
practically  all  heating  is  due  to  the  rarefied  gas.  Here  the  gas 
might  only  act  as  a  conductor  of  no  impedance  diverting  the  cur- 
rent from  the  wire  as  the  impedance  of  the  latter  is  enormously 
increased,  and  merely  heating  the  ends  of  the  wire  by  reason  of 
their  resistance  to  the  passage  of  the  discharge.  But  it  is  not 
at  all  necessary  that  the  gas  in  the  tube  should  be  conducting ;  it 
might  be  at  an  extremely  low  pressure,  still  the  ends  of  the  wire 
would  be  heated — as,  however,  is  ascertained  by  experience — 
only  the  two  ends  would  in  such  case  not  be  electrically  con- 
nected through  the  gaseous  medium.  Now  what  with  these  fre- 
quencies and  potentials  occurs  in  an  exhausted  tube,  occurs  in  the 
lightning  discharges  at  ordinary  pressure.  We  only  need  re- 
member one  of  the  facts  arrived  at  in  the  course  of  these  investi- 
gations, namely,  that  to  impulses  of  very  high  frequency  the  gas 
at  ordinary  pressure  behaves  much  in  the  same  manner  as  though 
it  were  at  moderately  low  pressure.  I  think  that  in  lightning 
discharges  frequently  wires  or  conducting  objects  are  volatilized 
merely  because  air  is  present,  and  that,  were  the  conductor  im- 


882  INVENTIONS  OF  NIKOLA  TESLA. 

merged  in  an  insulating  liquid,  it  would  be  safe,  for  then  the 
energy  would  have  to  spend  itself  somewhere  else.  From  the 
behavior  of  gases  under  sudden  impulses  of  high  potential,  I  am 
led  to  conclude  that  there  can  be  no  surer  way  of  diverting  a 
lightning  discharge  than  by  affording  it  a  passage  through  a 
volume  of  gas,  if  such  a  thing  can  be  done  in  a  practical  manner. 

There  are  two  more  features  upon  which  I  think  it  necessary 
to  dwell  in  connection  with  these  experiments — the  "  radiant 
state  "  and  the  "  non-striking  vacuum." 

Any  one  who  has  studied  Crookes'  work  must  have  received 
the  impression  that  the  "  radiant  state  "  is  a  property  of  the  gas 
inseparably  connected  with  an  extremely  high  degree  of  ex- 
haustion. But  it  should  be  remembered  that  the  phenomena 
observed  in  an  exhausted  vessel  are  limited  to  the  character  and 
capacity  of  the  apparatus  which  is  made  use  of.  1  think  that  in 
a  bulb  a  molecule,  or  atom,  does  not  precisely  move  in  a  straight 
line  because  it  meets  no  obstacle,  but  because  the  velocity  im- 
parted to  it  is  sufficient  to  propel  it  in  a  sensibly  straight  line. 
The  mean  free  path  is  one  thing,  but  the  velocity — the  energy 
associated  with  the  moving  body — is  another,  and  under  ordinary 
circumstances  I  believe  that  it  is  a  mere  question  of  potential  or 
speed.  A  disruptive  discharge  coil,  when  the  potential  is  pushed 
very  far,  excites  phosphorescence  and  projects  shadows,  at  com- 
paratively low  degrees  of  exhaustion.  In  a  lightning  discharge, 
matter  moves  in  straight  lines  at  ordinary  pressure  when  the 
mean  free  path  is  exceedingly  small,  and  frequently  images  of 
wires  or  other  metallic  objects  have  been  produced  by  the  par- 
ticles thrown  off  in  straight  lines. 

I  have  prepared  a  bulb  to  illustrate  by  an  experiment  the 
correctness  of  these  assertions.  In  a  globe  L,  Fig.  160,  I  have 
mounted  upon  a  lamp  filament  f  a  piece  of  lime  /.  The  lamp 
filament  is  connected  with  a  wire  which  leads  into  the  bulb,  and 
the  general  construction  of  the  latter  is  as  indicated  in  Fig.  148, 
before  described.  The  bulb  being  suspended  from  a  wire 
connected  to  the  terminal  of  the  coil,  and  the  latter  being  set  to 
work,  the  lime  piece  I  and  the  projecting  parts  of  the  filament  f 
are  bombarded.  The  degree  of  exhaustion  is  just  such  that  with 
the  potential  the  coil  is  capable  of  giving,  phosphorescence  of  the 
glass  is  produced,  but  disappears  as  soon  as  the  vacuum  is  im- 
paired. The  lime  containing  moisture,  and  moisture  being  given 
off  as  soon  as  heating  occurs,  the  phosphorescence  lasts  only  for 


HIGH  FREQUENCY  AND  HIGH  POTENTIAL  CURRENTS.      283 

a  few  moments.  When  the  lime  has  been  sufficiently  heated, 
enough  moisture  has  been  given  oft'  to  impair  materially  the 
vacuum  of  the  bulb.  As  the  bombardment  goes  on,  one  point 
of  the  lime  piece  is  more  heated  than  other  points,  and  the  result 
is  that  finally  practically  all  the  discharge  passes  through  that 
point  which  is  intensely  heated,  and  a  white  stream  of  lime  par- 
ticles (Fig.  160)  then  breaks  forth  from  that  point.  This  stream 
is  composed  of  "  radiant "  matter,  yet  the  degree  of  exhaustion 
is  low.  But  the  particles  move  in  straight  lines  because  the 
velocity  imparted  to  them  is  great,  and  this  is  due  to  three 
causes — to  the  great  electric  density,  the  high  temperature  of  the 
small  point,  and  the  fact  that  the  particles  of  the  lime  are  easily 


FIG.  160. 


torn  and  thrown  off — far  more  easily  than  those  of  carbon.  With 
frequencies  such  as  we  are  able  to  obtain,  the  particles  are  bodily 
thrown  off  and  projected  to  a  considerable  distance ;  but  with 
sufficiently  high  frequencies  no  such  thing  would  occur  ;  in  such 
case  only  a  stress  would  spread  or  a  vibration  would  be  propa- 
gated through  the  bulb.  It  would  be  out  of  the  question  to 
reacli  any  such  frequency  on  the  assumption  that  the  atoms  move 
with  the  speed  of  light ;  but  I  believe  that  such  a  thing  is  impos- 
sible ;  for  this  an  enormous  potential  would  be  required. 
With  potentials  which  we  are  able  to  obtain,  even  with  a  disrup- 
tive discharge  coil,  the  speed  must  be  quite  insignificant. 

As  to  the  "  non-striking  vacuum,"  the  point  to  be  noted  is, 
that   it   can   occur  only  with  low  frequency  impulses,  and  it  is 


284 


INVENTIONS  OF  NIKOLA  TKSLA. 


necessitated  by  the  impossibility  of  carrying  off  enough  energy 
with  such  impulses  in  high  vacuum,  since  the  few  atoms  which 
are  around  the  terminal  upon  coining  in  contact  with  the  same, 
are  repelled  and  kept  at  a  distance  for  a  comparatively  long 
period  of  time,  and  not  enough  work  can  be  performed  to  render 
the  effect  perceptible  to  the  eye.  If  the  difference  of  potential 
between  the  terminals  is  raised,  the  dielectric  breaks  down.  But 
with  very  high  frequency  impulses  there  is  no  necessity  for  such 
breaking  down,  since  any  amount  of  work  can  be  performed  by 
continually  agitating  the  atoms  in  the  exhausted  vessel,  provided 
the  frequency  is  higli  enough.  It  is  easy  to  reach — even  with 


FIG.  161. 


FIG.  162. 


frequencies  obtained  from  an  alternator  as  here  used — a  stage  at 
which  the  discharge  does  not  pass  between  two  electrodes  in  a 
narrow  tube,  each  of  these  being  connected  to  one  of  the  termi- 
nals of  the  coil,  but  it  is  difficult  to  reach  a  point  at  which  a 
luminous  discharge  would  not  occur  around  each  electrode. 

A  thought  which  naturally  presents  itself  in  connection  with 
high  frequency  currents,  is  to  make  use  of  their  powerful  electro- 
dynamic  inductive  action  to  produce  light  effects  in  a  sealed  glass 
globe.  The  leading-in  wire  is  one  of  the  defects  of  the  present 
incandescent  lamp,  and  if  no  other  improvement  were  made, 
that  imperfection  at  least  should  be  done  away  with.  Following 


HIGH  FREQUENCY  AND  1110 U  POTENTIAL  CURRENTS.     285 

this  thought,  I  have  carried  on  experiments  in  various  directions, 
of  which  some  were  indicated  in  my  former  paper.  I  may  here 
mention  one  or  two  more  lines  of  experiment  which  have  been 
followed  up. 

Many  bulbs  were  constructed  as  shown  in  Fig.  161  and  Fig. 
162. 

In  Fig.  161,  a  wide  tube,  T,  was  sealed  to  a  smaller  W  shaped 
tube  u,  of  phosphorescent  glass.  In  the  tube  T,  was  placed  a  coil 
c,  of  aluminum  wire,  the  ends  of  which  were  provided  with 
small  spheres,  t  and  ^,  of  aluminum,  and  reached  into  the  u  tube. 
The  tube  T  was  slipped  into  a  socket  containing  a  primary  coil, 
through  which  usually  the  discharges  of  Leyden  jars  were  di- 
rected, and  the  rarefied  gas  in  the  small  u  tube  was  excited  to 
strong  luminosity  by  the  high-tension  current  induced  in  the  coil  c. 
When  Leyden  jar  discharges  were  used  to  induce  currents  in  the 
coil  c,  it  was  found  necessary  to  pack  the  tube  T  tightly  with  in- 
sulating powder,  as  a  discharge  would  occur  frequently  between 
the  turns  of  the  coil,  especially  when  the  primary  was  thick  and 
the  air  gap,  through  which  the  jars  discharged,  large,  and  no 
little  trouble  was  experienced  in  this  way. 

In  Fig.  162  is  illustrated  another  form  of  the  bulb  constructed. 
In  this  case  a  tube  T  is  sealed  to  a  globe  L.  The  tube  contains  a 
coil  c,  the  ends  of  which  pass  through  two  small  glass  tubes  t 
and  ti,  which  are  sealed  to  the  tube  T.  Two  refractory  buttons 
m  and  mt  are  mounted  on  lamp  filaments  which  are  fastened  to 
the  ends  of  the  wires  passing  through  the  glass  tubes  t  and  £,. 
Generally  in  bulbs  made  on  this  plan  the  globe  L  communicated 
with  the  tube  T.  For  this  purpose  the  ends  of  the  small  tubes  t 
and  ti  were  heated  just  a  trifle  in  the  burner,  merely  to  hold  the 
wires,  but  not  to  interfere  with  the  communication.  The  tube  T, 
with  the  small  tubes,  wires  through  the  same,  and  the  refractory 
buttons  in  and  ml9  were  first  prepared,  and  then  sealed  to  globe  L, 
whereupon  the  coil  c  was  slipped  in  and  the  connections  made  to 
its  ends.  The  tube  was  then  packed  with  insulating  powder, 
jamming  the  latter  as  tight  as  possible  up  to  very  nearly  the  end  ; 
then  it  was  closed  and  only  a  small  hole  left  through  which  the 
remainder  of  the  powder  was  introduced,  and  finally  the  end  of 
the  tube  was  closed.  Usually  in  bulbs  constructed  as  shown  in 
Fig.  162  an  aluminum  tube  a  was  fastened  to  the  upper  end  s 
of  each  of  the  tubes  t  and  £b  in  order  to  protect  that  end  against 
the  heat.  The  buttons  m  and  m^  could  be  brought  to  any  degree 


286  INVENTIONS  OF  NIKOLA  TESLA. 

of  incandescence  by  passing  the  discharges  of  Leyden  jars 
around  the  coil  c.  In  such  bulbs  with  two  buttons  a  very  curi- 
ous effect  is  produced  by  the  formation  of  the  shadows  of  each 
of  the  two  buttons. 

Another  line  of  experiment,  which  has  been  assiduously  fol- 
lowed, was  to  induce  by  electro-dynamic  induction  a  current  or 
luminous  discharge  in  an  exhausted  tube  or  bulb.  This  matter 
has  received  such  able  treatment  at  the  hands  of  Prof.  J.  J. 
Thomson,  that  I  could  add  but  little  to  what  he  has  made  known, 
even  had  I  made  it  the  special  subject  of  this  lecture.  Still, 
since  experiments  in  this  line  have  gradually  led  me  to  the  pres- 
ent views  and  results,  a  few  words  must  be  devoted  here  to  this 
subject. 

It  has  occured,  no  doubt,  to  many  that  as  a  vacuum  tube  is 
made  longer,  the  electromotive  force  per  unit  length  of  the  tube, 
necessary  to  pass  a  luminous  discharge  through  the  latter,  becomes 
continually  smaller ;  therefore,  if  the  exhausted  tube  be  made 
long  enough,  even  with  low  frequencies  a  luminous  discharge 
could  be  induced  in  such  a  tube  closed  upon  itself.  Such  a  tube 
might  be  placed  around  a  hall  or  on  a  ceiling,  and  at  once  a  sim- 
ple appliance  capable  of  giving  considerable  light  would  be  ob- 
tained. But  this  would  be  an  appliance  hard  to  manufacture 
and  extremely  unmanageable.  It  would  not  do  to  make  the 
tube  up  of  small  lengths,  because  there  would  be  with  ordinary 
frequencies  considerable  loss  in  the  coatings,  and  besides,  if  coat- 
ings were  used,  it  would  be  better  to  supply  the  current  directly 
to  the  tube  by  connecting  the  coatings  to  a  transformer.  But 
even  if  all  objections  of  such  nature  were  removed,  with 
low  frequencies  the  light  conversion  itself  would  be  inefficient, 
as  I  have  before  stated.  In  using  extremely  high  frequencies 
the  length  of  the  secondary — in  other  words,  the  size  of  the  ves- 
sel— can  be  reduced  as  much  as  desired,  and  the  efficiency  of  the 
light  conversion  is  increased,  provided  that  means  are  invented 
for  efficiently  obtaining  such  high  frequencies.  Thus  one  is  led, 
from  theoretical  and  practical  considerations,  to  the  use  of  high 
frequencies,  and  this  means  high  electromotive  forces  and  small 
currents  in  the  primary.  When  one  works  with  condenser 
charges — and  they  are  the  only  means  up  to  the  present  known 
for  reaching  these  extreme  frequencies — one  gets  to  electromotive 
forces  of  several  thousands  of  volts  per  turn  of  the  primary.  "We 
cannot  multiply  the  electro-dynamic  inductive  effect  by  taking 


man  FREQUENCY  AND  HIGH  POTENTIAL  CURRENTS.    287 

more  turns  in  the  primary,  for  we  arrive  at  the  conclusion  that 
the  best  way  is  to  work  with  one  single  turn — though  we  must 
sometimes  depart  from  this  rule — and  we  must  get  along  with 
whatever  inductive  effect  we  can  obtain  with  one  turn.  But  be- 
fore one  has  long  experimented  with  the  extreme  frequencies  re- 
quired to  set  up  in  a  small  bulb  an  electromotive  force  of  several 
thousands  of  volts,  one  realizes  the  great  importance  of  electrosta- 
tic effects,  and  these  effects  grow  relatively  to  the  electro-dyna- 
mic in  significance  as  the  frequency  is  increased. 

Kow,  if  anything  is  desirable  in  this  case,  it  is  to  increase  the 
frequency,  and  this  would  make  it  still  worse  for  the  electro- 
dynamic  effects.  On  the  other  hand,  it  is  easy  to  exalt  the  elec- 
trostatic action  as  far  as  one  likes  by  taking  more  turns  on  the 
secondary,  or  combining  self-induction  and  capacity  to  raise  the 
potential.  It  should  also  be  remembered  that,  in  reducing  the 
the  current  to  the  smallest  value  and  increasing  the  potential, 
the  electric  impulses  of  high  frequency  can  be  more  easily  trans- 
mitted through  a  conductor. 

These  and  similar  thoughts  determined  me  to  devote  more  at 
tention  to  the  electrostatic  phenomena,  and  to  endeavor  to  pro- 
duce potentials  as  high  as  possible,  and  alternating  as  fast  as 
they  could  be  made  to  alternate.  I  then  found  that  I  could  ex- 
cite vacuum  tubes  at  considerable  distance  from  a  conductor 
connected  to  a  properly  constructed  coil,  and  that  I  could,  by 
converting  the  oscillatory  current  of  a  conductor  to  a  higher  po- 
tential, establish  electrostatic  alternating  fields  which  acted 
through  the  whole  extent  of  the  room,  lighting  up  a  tube  no 
matter  where  it  was  held  in  space.  I  thought  I  recognized  that 
I  had  made  a  step  in  advance,  and  I  have  persevered  in  this  line ; 
but  I  wish  to  say  that  I  share  with  all  lovers  of  science  and  pro- 
gress the  one  and  only  desire — to  reach  a  result  of  utility  to  men 
in  any  direction  to  which  thought  or  experiment  may  lead  me. 
I  think  that  this  departure  is  the  right  one,  for  I  cannot  see, 
from  the  observation  of  the  phenomena  which  manifest  them- 
selves as  the  frequency  is  increased,  what  there  would  remain  to 
act  between  two  circuits  conveying,  for  instance,  impulses  of 
several  hundred  millions  per  second,  except  electrostatic  forces. 
Even  with  such  trifling  frequencies  the  energy  would  be  practically 
all  potential,  and  my  conviction  has  grown  strong  that,  to  whatever 
kind  of  motion  light  may  be  due,  it  is  produced  by  tremendous 
electrostatic  stresses  vibrating  with  extreme  rapidity. 


288  INVENTIONS  OF  NIKOLA  TESLA. 

Of  all  these  phenomena  observed  with  currents,  or  electric 
impulses,  of  high  frequency,  the  most  fascinating  for  an  aud- 
ience are  certainly  those  which  are  noted  in  an  electrostatic  field 
acting  through  considerable  distance;  and  the  best  an  unskilled 
lecturer  can  do  is  to  begin  and  finish  with  the  exhibition  of  these 
singular  effects.  I  take  a  tube  in  my  hand  and  move  it  about, 
and  it  is  lighted  wherever  I  may  hold  it;  throughout  space  the 
invisible  forces  act.  But  I  may  take  another  tube  and  it  might 
not  light,  the  vacuum  being  very  high.  I  excite  it  by  means  of  a 
disruptive  discharge  coil,  and  now  it  will  light  in  the  electrostatic 


FIG.  163.  FIG.  164. 

field.  I  may  put  it  away  for  a  few  weeks  or  months,  still  it  retains 
the  faculty  of  being  excited.  What  change  have  I  produced  in  the 
tube  in  the  act  of  exciting  it?  If  a  motion  imparted  to  atoms,  it 
is  difficult  to  perceive  how  it  can  persist  so  long  without  being 
arrested  by  f  rictional  losses ;  and  if  a  strain  exerted  in  the  dielec- 
tric, such  as  a  simple  electrification  would  produce,  it  is  easy  to 
see  how  it  may  persist  indefinitely,  but  very  difficult  to  under- 
stand why  such  a  condition  should  aid  the  excitation  when  we 
have  to  deal  with  potentials  which  are  rapidly  alternating. 


HIGH  FREQUENCY  AND  HIGH  POTENTIAL  CURRENTS.      289 

Since  I  have  exhibited  these  phenomena  for  the  first  time,  I 
have  obtained  some  other  interesting  effects.  For  instance,  I 
have  produced  the  incandescence  of  a  button,  filament,  or  wire 
enclosed  in  a  tube.  To  get  to  this  result  it  was  necessary  to 
economize  the  energy  which  is  obtained  from  the  field,  and  direct 
most  of  it  on  the  small  body  to  be  rendered  incandescent.  At 
the  beginning  the  task  appeared  difficult,  but  the  experiences 
gathered  permitted  me  to  reach  the  result  easily.  In  Fig.  163 
and  Fig.  164,  two  such  tubes  are  illustrated,  which  are  prepared  for 
the  occasion.  In  Fig.  163  a  short  tube  TJ,  sealed  to  another  long 
tube  T,  is  provided  with  a  stem  .$,  with  a  platinum  wire  sealed  in 
the  latter.  A  very  thin  lamp  filament  I,  is  fastened  to  this  wire 
and  connection  to  the  outside  is  made  through  a  thin  copper  wire 
w.  The  tube  is  provided  with  outside  and  inside  coatings,  c  and 
GJ,  respectively,  and  is  filled  as  far  as  the  coatings  reach  with  con- 
ducting, and  the  space  above  with  insulating,  powder.  These 
coatings  are  merely  used  to  enable  me  to  perform  two  experi- 
ments with  the  tube — namely,  to  produce  the  effect  desired  either 
by  direct  connection  of  the  body  of  the  experimenter  or  of  an- 
other body  to  the  wire  w,  or  by  acting  inductively  through  the 
glass.  The  stem  s  is  provided  with  an  aluminum  tube  «,  for 
purposes  before  explained,  and  only  a  small  part  of  the  filament 
reaches  out  of  this  tube.  By  holding  the  tube  T:  anywhere  in 
the  electrostatic  field,  the  filament  is  rendered  incandescent. 

A  more  interesting  piece  of  apparatus  is  illustrated  in  Fig.  164. 
The  construction  is  the  same  as  before,  only  instead  of  the  lamp 
filament  a  small  platinum  wire  ^>,  sealed  in  a  stem  s,  and  bent 
above  it  in  a  circle,  is  connected  to  the  copper  wire  w,  which  is 
joined  to  an  inside  coating  c.  A  small  stem  *M  is  provided  with 
a  needle,  on  the  point  of  which  is  arranged,  to  rotate  very  freely, 
a  very  light  fan  of  mica  v.  To  prevent  the  fan  from  falling  out, 
a  thin  stem  of  glass  </,  is  bent  properly  and  fastened  to  the  alu- 
minum tube.  When  the  glass  tube  is  held  anywhere  in  the  elec- 
trostatic field  the  platinum  wire  becomes  incandescent,  and  the 
mica  vanes  are  rotated  very  fast. 

Intense  phosphorescence  may  be  excited  in  a  bulb  by  merely 
connecting  it  to  a  plate  within  the  field,  and  the  plate  need  not 
be  any  larger  than  an  ordinary  lamp  shade.  The  phosphores- 
cence excited  with  these  currents  is  incomparably  more  powerful 
than  with  ordinary  apparatus.  A  small  phosphorescent  bulb, 
when  attached  to  a  wire  connected  to  a  coil,  emits  sufficient  light 


290  INVENTIONS  OF  NIKOLA  TESLA. 

to  allow  reading  ordinary  print  at  a  distance  of  five  to  six  paces. 
It  was  of  interest  to  see  how  some  of  the  phosphorescent  bulbs 
of  Professor  Crookes  would  behave  with  these  currents,  and  he 
has  had  the  kindness  to  lend  me  a  few  for  the  occasion.  The 
effects  produced  are  magnificent,  especially  by  the  sulphide  of 
calcium  and  sulphide  of  zinc.  With  the  disruptive  discharge 
coil  they  glow  intensely  merely  by  holding  them  in  the  hand  and 
connecting  the  body  to  the  terminal  of  the  coil. 

To  whatever  results  investigations  of  this  kind  may  lead,  the 
chief  interest  lies,  for  the  present,  in  the  possibilities  they  offer 
for  the  production  of  an  efficient  illuminating  device.  In  no 
branch  of  electric  industry  is  an  advance  more  desired  than  in 
the  manufacture  of  light.  Every  thinker,  when  considering  the 
barbarous  methods  employed,  the  deplorable  losses  incurred  in 
our  best  systems  of  light  production,  must  have  asked  himself, 
What  is  likely  to  be  the  light  of  the  future  ?  Is  it  to  be  an  in- 
candescent solid,  as  in  the  present  lamp,  or  an  incandescent  gas, 
or  a  phosphorescent  body,  or  something  like  a  burner,  but  in- 
comparably more  efficient  ? 

There  is  little  chance  to  perfect  a  gas  burner ;  not,  perhaps, 
because  human  ingenuity  has  been  bent  upon  that  problem  for 
centuries  without  a  radical  departure  having  been  made — 
though  the  argument  is  not  devoid  of  force — but  because  in  a 
burner  the  highest  vibrations  can  never  be  reached,  except  by 
passing  through  all  the  low  ones.  For  how  is  a  flame  to  proceed 
unless  by  a  fall  of  lifted  weights  ?  Such  process  cannot  be  main- 
tained without  renewal,  and  renewal  is  repeated  passing  from  low 
to  high  vibrations.  One  way  only  seems  to  be  open  to  improve 
a  burner,  and  that  is  by  trying  to  reach  higher  degrees  of  incan- 
descence. Higher  incandescence  is  equivalent  to  a  quicker  vi- 
bration :  that  means  more  light  from  the  same  material,  and  that 
again,  means  niore  economy.  In  this  direction  some  improve- 
ments have  been  made,  but  the  progress  is  hampered  by  many 
limitations.  Discarding,  then,  the  burner,  there  remains  the 
three  ways  first  mentioned,  which  are  essentially  electrical. 

Suppose  the  light  of  the  immediate  future  to  be  a  solid,  ren- 
dered incandescent  by  electricity.  Would  it  not  seem  that  it  is 
better  to  employ  a  small  button  than  a  frail  filament  ?  From 
many  considerations  it  certainly  must  be  concluded  that  a  button 
is  capable  of  a  higher  economy,  assuming,  of  course,  the  diffi- 
culties connected  with  the  operation  of  such  a  lamp  to  be  effec- 


HIGH  FRKQUENCY  AND  HIGH  POTENTIAL  CURRENTS.      291 

lively  overcome.  But  to  light  such  a  lamp  we  require  a  high 
potential ;  and  to  get  this  economically,  we  must  use  high  fre- 
quencies. 

Such  considerations  apply  even  more  to  the  production  of  light 
by  the  incandescence  of  a  gas,  or  by  phosphorescence.  In  all 
cases  we  require  high  frequencies  and  high  potentials.  These 
thoughts  occurred  to  me  a  long  time  ago. 

Incidentally  we  gain,  by  the  use  of  high  frequencies,  many  ad- 
vantages, such  as  higher  economy  in  the  light  production,  the 
possibility  of  working  with  one  lead,  the  possibility  of  doing  away 
with  the  leading-in  wire,  etc. 

The  question  is,  how  far  can  we  go  with  frequencies  ?  Ordi- 
nary conductors  rapidly  lose  the  facility  of  transmitting  electric 
impulses  when  the  frequency  is  greatly  increased.  Assume  the 
means  for  the  production  of  impulses  of  very  great  frequency 
brought  to  the  utmost  perfection,  every  one  will  naturally  ask 
how  to  transmit  them  when  the  necessity  arises.  In  transmitting 
such  impulses  through  conductors  we  must  remember  that  we 
have  to  deal  with  pressure  and  flow,  in  the  ordinary  interpretation 
of  these  terms.  Let  the  pressure  increase  to  an  enormous  value, 
and  let  the  flow  correspondingly  diminish,  then  such  impulses — 
variations  merely  of  pressure,  as  it  were — can  no  doubt  be 
transmitted  through  a  wire  even  if  their  frequency  be  many 
hundreds  of  millions  per  second.  It  would,  of  course,  be  out  of 
question  to  transmit  such  impulses  through  a  wire  immersed  in  a 
gaseous  medium,  even  if  the  wire  were  provided  with  a  thick 
and  excellent  insulation,  for  most  of  the  energy  would  be  lost  in 
molecular  bombardment  and  consequent  heating.  The  end  of 
the  wire  connected  to  the  source  would  be  heated,  and  the  re- 
mote end  would  receive  but  a  trifling  part  of  the  energy  sup- 
plied. The  prime  necessity,  then,  if  such  electric  impulses  are 
to  be  used,  is  to  find  means  to  reduce  as  much  as  possible  the 
dissipation. 

The  first  thought  is,  to  employ  the  thinnest  possible  wire  sur- 
rounded by  the  thickest  practicable  insulation.  The  next  thought 
is  to  employ  electrostatic  screens.  The  insulation  of  the  wire 
may  be  covered  with  a  thin  conducting  coating  and  the  latter 
connected  to  the  ground.  But  this  would  not  do,  as  then  all  the 
energy  would  pass  through  the  conducting  coating  to  the  ground 
and  nothing  would  get  to  the  end  of  the  wire.  If  a  ground  con- 
nection is  made  it  can  only  be  made  through  a  conductor  offer- 


292  INVENTIONS  OF  NIKOLA  TESLA. 

ing  an  enormous  impedance,  or  through  a  condenser  of  ex- 
tremely small  capacity.  This,  however,  does  not  do  away  with 
other  difficulties. 

If  the  wave  length  of  the  impulses  is  much  smaller  than  the 
length  of  the  wire,  then  corresponding  short  waves  will  be  set 
up  in  the  conducting  coating,  and  it  will  be  more  or  less  the 
same  as  though  the  coating  were  directly  connected  to  earth.  It 
is  therefore  necessary  to  cut  up  the  coating  in  sections  much 
shorter  than  the  wave  length.  Such  an  arrangement  does  not 
still  afford  a  perfect  screen,  but  it  is  ten  thousand  times  better 
than  none.  I  think  it  preferable  to  cut  up  the  conducting  coat- 
ing in  small  sections,  even  if  the  current  waves  be  much  longer 
than  the  coating. 

If  a  wire  were  provided  with  a  perfect  electrostatic  screen,  it 
would  be  the  same  as  though  all  objects  were  removed  from  it  at 
infinite  distance.  The  capacity  would  then  be  reduced  to  the 
capacity  of  the  wire  itself,  which  would  be  very  small.  It 
would  then  be  possible  to  send  over  the  wire  current  vibrations 
of  very  high  frequencies  at  enormous  distances,  without  affecting 
greatly  the  character  of  the  vibrations.  A  perfect  screen  is  of 
course  out  of  the  question,  but  I  believe  that  with  a  screen  such 
as  I  have  just  described  telephony  could  be  rendered  practicable 
across  the  Atlantic.  According  to  my  ideas,  the  gutta-percha 
covered  wire  should  be  provided  with  a  third  conducting  coating 
subdivided  in  sections.  On  the  top  of  this  should  be  again 
placed  a  layer  of  gutta-percha  and  other  insulation,  and  on  the 
top  of  the  whole  the  armor.  But  such  cables  will  not  be  con- 
structed, for  ere  long  intelligence — transmitted  without  wires — 
will  throb  through  the  earth  like  a  pulse  through  a  living  organ- 
ism. The  wonder  is  that,  with  the  present  state  of  knowledge 
and  the  experiences  gained,  no  attempt  is  being  made  to  dis- 
turb the  electrostatic  or  magnetic  condition  of  the  earth,  and 
transmit,  if  nothing  else,  intelligence. 

It  has  been,  my  chief  aim  in  presenting  these  results  to  point 
out  phenomena  or  features  of  novelty,  and  to  advance  ideas 
which  I  am  hopeful  will  serve  as  starting  points  of  new  depart- 
ures. It  has  been  my  chief  desire  this  evening  to  entertain  you 
with  some  novel  experiments.  Your  applause,  so  frequently 
and  generously  accorded,  has  told  me  that  I  have  succeeded. 

In  conclusion,  let  me  thank  you  most  heartily  for  your  kind- 
ness and  attention,  and  assure  you  that  the  honor  I  have  had  in 


HJGH  FREQUENCY  AND  HIGH  POTENTIAL  CURRENTS.      293 

addressing  such  a  distinguished  audience,  the  pleasure  I  have  had 
in  presenting  these  results  to  a  gathering  of  so  many  able  men — 
and  among  them  also  some  of  those  in  whose  work  for  many 
years  past  I  have  found  enlightenment  and  constant  pleasure — 
I  shall  never  forget. 


CHAPTER  XXVIII. 
ON  LIGHT  AND  OTHER  HIGH  FREQUENCY  PHENOMENA.1 

INTRODUCTORY. SOME  THOUGHTS  ON  THE  EYE. 

WHEN  we  look  at  the  world  around  us,  on  Nature,  we  are  im- 
pressed with  its  beauty  and  grandeur.  Each  thing  we  perceive, 
though  it  may  be  vanishingly  small,  is  in  itself  a  world,  that  is, 
like  the  whole  of  the  universe,  matter  and  force  governed  by 
law, — a  world,  the  contemplation  of  which  fills  us  with  feelings 
of  wonder  and  irresistibly  urges  us  to  ceaseless  thought  and  in- 
quiry. But  in  all  this  vast  world,  of  all  objects  our  senses  re- 
veal to  us,  the  most  marvellous,  the  most  appealing  to  our 
imagination,  appears  no  doubt  a  highly  developed  organism,  a 
thinking  being.  If  there  is  anything  fitted  to  make  us  admire 
Nature's  handiwork,  it  is  certainly  this  inconceivable  structure, 
which  performs  its  innumerable  motions  of  obedience  to  external 
influence.  To  understand  its  workings,  to  get  a  deeper  insight 
into  this  Nature's  masterpiece,  has  ever  been  for  thinkers  a  fascin- 
ating aim,  and  after  many  centuries  of  arduous  research  men  have 
arrived  at  a  fair  understanding  of  the  functions  of  its  organs  and 
senses.  Again,  in  all  the  perfect  harmony  of  its  parts,  of  the 
parts  which  constitute  the  material  or  tangible  of  our  being,  of  all 
its  organs  and  senses,  the  eye  is  the  most  wonderful.  It  is  the 
most  precious,  the  most  indispensable  of  our  perceptive  or  direct- 
ive organs,  it  is  the  great  gateway  through  which  all  knowledge 
enters  the  mind.  Of  all  our  organs,  it  is  the  one,  which  is  in  the 

1.  A  lecture  delivered  before  the  Franklin  Institute,  Philadelphia,  February* 
1893,  and  before  the  National  Electric  Light  Association,  St.  Louis,  March, 


HIGH  FREQUENCY  AND  HIGH  POTENTIAL  CURRENTS.      295 

most  intimate  relation  with  that  which  we  call  intellect.  So  inti- 
mate is  this  relation,  that  it  is  often  said,  the  very  soul  shows 
itself  in  the  eye. 

It  can  he  taken  as  a  fact,  which  the  theory  of  the  action  of  the 
eye  implies,  that  for  each  external  impression,  that  is,  for  each 
image  produced  upon  the  retina,  the  ends  of  the  visual  nerves, 
concerned  in  the  conveyance  of  the  impression  to  the  mind,  must 
be  under  a  peculiar  stress  or  in  a  vibratory  state.  It  now  does 
not  seem  improbable  that,  when  by  the  power  of  thought  an  im- 
age is  evoked,  a  distinct  reflex  action,  no  matter  how  weak,  is 
exerted  upon  certain  ends  of  the  visual  nerves,  and  therefore 
upon  the  retina.  Will  it  ever  be  within  human  power  to  analyze 
the  condition  of  the  retina  when  disturbed  by  thought  or  reflex 
action,  by  the  help  of  some  optical  or  other  means  of  such  sensi- 
tiveness, that  a  clear  idea  of  its  state  might  be  gained  at  any 
time  2  If  this  were  possible,  then  the  problem  of  reading  cne's 
thoughts  with  precision,  like  the  characters  of  an  open  book, 
might  be  much  easier  to  solve  than  many  problems  belonging  to 
the  domain  of  positive  physical  science,  in  the  solution  of  which 
many,  if  not  the  majority,  of  scientific  men  implicitly  believe. 
Helmholtz,  has  shown  that  the  fundi  of  the  eye  are  themselves, 
luminous,  and  he  was  able  to  see,  in  total  darkness,  the  move- 
ment of  his  arm  by  the  light  of  his  own  eyes.  This  is  one  of  the 
most  remarkable  experiments  recorded  in  the  history  of  science, 
and  probably  only  a  few  men  could  satisfactorily  repeat  it,  for  it 
is  very  likely,  that  the  luminosity  of  the  eyes  is  associated  with 
uncommon  activity  of  the  brain  and  great  imaginative  power.  It 
is  fluorescence  of  brain  action,  as  it  were. 

Another  fact  having  a  bearing  on  this  subject  which  has  prob- 
ably been  noted  by  many,  since  it  is  stated  in  popular  expressions, 
but  which  I  cannot  recollect  to  have  found  chronicled  as  a  posi- 
tive result  of  observation  is,  that  at  times,  when  a  sudden  idea  or 
image  presents  itself  to  the  intellect,  there  is  a  distinct  and  some- 
times painful  sensation  of  luminosity  produced  in  the  eye,  ob- 
servable even  in  broad  daylight. 

The  saying  then,  that  the  soul  shows  itself  in  the  eye,  is  deep- 
ly founded,  and  we  feel  that  it  expresses  a  great  truth.  It  has  a 
profound  meaning  even  for  one  who,  like  a  poet  or  artist,  only 
following  his  inborn  instinct  or  love  for  Nature,  finds  delight  in 
aimless  thoughts  and  in  the  mere  contemplation  of  natural  phe- 
nomena, but  a  still  more  profound  meaning  for  one  who,  in  the 


290  INVENTIONS  OF  NIKOLA  TESLA. 

spirit  of  positive  scientific  investigation,  seeks  to  ascertain  the 
causes  of  the  effects.  It  is  principally  the  natural  philospher, 
the  physicist,  for  whom  the  eye  is  the  subject  of  the  most  intense 
admiration. 

Two  facts  about  the  eye  must  forcibly  impress  the  mind  of  the 
physicist,  notwithstanding  he  may  think  or  say  that  it  is  an 
imperfect  optical  instrument,  forgetting,  that  the  very  conception 
of  that  which  is  perfect  or  seems  so  to  him,  has  been  gained 
through  this  same  instrument.  First,  the  eye  is,  as  far  as  our 
positive  knowledge  goes,  the  only  organ  which  is  directly  affected 
by  that  subtile  medium,  which  as  science  teaches  us,  must  fill  all 
space  ;  secondly,  it  is  the  most  sensitive  of  our  organs,  incompar- 
ably more  sensitive  to  external  impressions  than  any  other. 

The  organ  of  hearing  implies  the  impact  of  ponderable  bodies, 
the  organ  of  smell  the  transference  of  detached  material  particles, 
and  the  organs  of  taste,  and  of  touch  or  force,  the  direct  contact, 
or  at  least  some  interference  of  ponderable  matter,  and  this  is 
true  even  in  those  instances  of  animal  organisms,  in  which  some 
of  these  organs  are  developed  to  a  degree  of  truly  marvelous 
perfection.     This  being  so,  it  seems  wonderful  that  the  organ  of 
,  ^'c      sight  solely  should  be  capable  of  being  stirred  by  that,  which  all 
•  •>„    our  other  organs  are  powerless  to  detect,  yet  which  plays  an  es- 
sential part  in  all  natural  phenomena,  which  transmits  all  energy 
and  sustains  all  motion  and,  that  most  intricate   of  all,  life,  but 
which  has  properties  such  that  even  a  scientifically  trained  mind 
cannot  help  drawing  a  distinction  between  it  and  all  that  is  called 
matter.     Considering  merely  this,  and  the  fact  that  the  eye,  by 
L    its  marvelous  power,  widens  our  otherwise  very  narrow  range  of 
«•'*'  •'  perception  far  beyond  the  limits  of  the  small  world  which  is  our 
own,  to  embrace  myriads  of  other  worlds,  suns  and  stars  in  the 
v  *  infinite  depths  of  the  universe,  would  make  it  justifiable  to  assert, 
.        that  it  is  an  organ  of  a  higher  order.    Its  performances  are  beyond 
->.  comprehension.     Nature  as  far  as  we  know  never  produced  any- 
t  *t«    thing  more  wonderful.     We  can  get  barely  a  faint  idea  of  its 
',         prodigious  power  by  analyzing  what  it  does  and  by  comparing. 
When  ether  waves  impinge  upon  the  human  body,  they  produce 
the  sensations  of  warmth  or  cold,  pleasure  or  pain,  or  perhaps  other 
#v~a  sensations  of  which  we  are  not  aware,  and  any  degree  or  intensity 
£/^cWof  these  sensations,  which  degrees  are  infinite  in  number,  hence  an 
infinite  number  of  distinct  sensations.     But  our  sense  of  touch,  or 
our  sense  of  force,  cannot  reveal  to  us  these  differences  in  degree 

' 


HIGH  FREQUENCY  AND  HIGH  POTENTIAL  CURRENTS.      297 

or  intensity,  unless  they  are  very  great.  Now  we  can  readily  con- 
ceive how  an  organism,  such  as  the  human,  in  the  eternal  process 
of  evolution,  or  more  philosophically  speaking,  adaptation  to 
Nature,  being  constrained  to  the  use  of  only  the  sense  of  touch  or 
force,  for  instance,  might  develop  this  sense  to  such  a  degree  of 
senstiveness  or  perfection,  that  it  would  be  capable  of  distinguish- 
ing the  minutest  differences  in  the  temperature  of  a  body  even 
at  some  distance,  to  a  hundredth,  or  thousandth,  or  millionth  part 
of  a  degree.  Yet,  even  this  apparently  impossible  performance 
would  not  begin  to  compare  with  that  of  the  eye,  which  is  cap- 
able of  distinguishing  and  conveying  to  the  mind  in  a  single 
instant  innumerable  peculiarities  of  the  body,  be  it  in  form, 
or  color,  or  other  respects.  This  power  of  the  eye  rests  upon 
two  things,  namely,  the  rectilinear  propagation  of  the  disturb- 
ance by  which  it  is  effected,  and  upon  its  sensitiveness. 
To  say  that  the  eye  is  sensitive  is  not  saying  anything.  Compared 
with  it,  all  other  organs  are  monstrously  crude.  The  organ  of 
smell  which  guides  a  dog  on  the  trail  of  a  deer,  the  organ  of  touch 
or  force  which  guides  an -insect  in  its  wanderings,  the  organ  of 
hearing,  which  is  affected  by  the  slightest  disturbances  of  the  air, 
are  sensitive  organs,  to  be  sure,  but  what  are  they  compared  with 
the  human  eye !  No  doubt  it  responds  to  the  faintest  echoes  or 
r 3 ve liberations  of  the  medium  ;  no  doubt,  it  brings  us  tidings  from 
other  worlds,  infinitely  remote,  but  in  a  language  we  cannot  as 
yet  always  understand.  And  why  not  ?  Because  we  live  in  a 
medium  filled  with  air  and  other  gases,  vapors  and  a  dense  mass 
of  solid  particles  flying  about.  These  play  an  important  part  in 
many  phenomena ;  they  fritter  away  the  energy  of  the  vibrations 
before  they  can  reach  the  eye ;  they  too,  are  the  carriers  of  germs 
of  destruction,  they  get  into  our  lungs  and  other  organs,  clog  up 
the  channels  and  imperceptibly,  yet  inevitably,  arrest  the  stream 
of  life.  Could  we  but  do  away  with  all  ponderable  matter  in  the 
line  of  sight  of  the  telescope,  it  would  reveal  to  us  undreamt  of 
marvels.  Even  the  unaided  eye,  I  think,  would  be  capable  of  dis- 
tinguishing in  the  pure  medium,  small  objects  at  distances  meas- 
ured probably  by  hundreds  or  perhaps  thousands  of  miles. 

But  there  is  something  else  about  the  eye  which  impresses  us 
still  more  than  these  wonderful  features  which  we  observed,  view- 
ing it  from  the  standpoint  of  a  physicist,  merely  as  an  optical 
instrument, — something  which  appeals  to  us  more  than  its  marvel- 
ous faculty  of  being  directly  affected  by  the  vibrations  of  the 


298  INVENTIONS  OF  NIKOLA  TESLA. 

medium,  without  interference  of  gross  matter,  and  more  than  its 
inconceivable  sensitiveness  and  discerning  power.  It  is  its  sig- 
nificance in  the  processes  of  life.  No  matter  what  one's  views  oh 
nature  and  life  may  be,  he  must  stand  amazed  when,  for  the  first 
time  in  his  thoughts,  he  realizes  the  importance  of  the  eye  in  the 
physical  processes  and  mental  performances  of  the  human  organ- 
ism. And  how  could  it  be  otherwise,  when  he  realizes,  that  the 
eye  is  the  means  through  which  the  human  race  has  acquired 
the  entire  knowledge  it  possesses,  that  it  controls  all  our  motions, 
more  still,  all  our  actions. 

There  is  no  way  of  acquiring  knowledge  except  through  the  eye. 
What  is  the  foundation  of  all  philosophical  systems  of  ancient 
and  modern  times,  in  fact,  of  all  the  philosophy  of  man  ?  /  am, 
I  think  •  I  think,  therefore  Iain.  But  how  could  I  think  and  how 
would  I  know  that  I  exist,  if  I  had  not  the  eye  ?  For  knowledge 
involves  consciousness  ;  consciousness  involves  ideas,  conceptions  ; 
conceptions  involve  pictures  or  images,  and  images  the  sense  of 
vision,  and  therefore  the  organ  of  sight.  But  how  about  blind 
men,  will  be  asked  ?  Yes,  a  blind  man  may  depict  in  magnificent 
poems,  forms  and  scenes  from  real  life,  from  a  world  he  physically 
does  not  see.  A  blind  man  may  touch  the  keys  of  an  instrument 
with  unerring  precision,  may  model  the  fastest  boat,  may  discover 
and  invent,  calculate  and  construct,  may  do  still  greater  wonders — 
but  all  the  blind  men  who  have  done  such  things  have  descended 
from  those  who  had  seeing  eyes.  Nature  may  reach  the  same  re- 
sult in  many  ways.  Like  a  wave  in  the  physical  world,  in  the  in- 
finite ocean  of  the  medium  which  pervades  all,  so  in  the  world  of 
organisms,  in  life,  an  impulse  started  proceeds  onward,  at  times, 
may  be,  with  the  speed  of  light,  at  times,  again,  so  slowly  that 
for  ages  and  ages  it  seems  to  stay,  passing  through  processes  of  a 
complexity  inconceivable  to  men,  but  in  all  its  forms,  in  all  its 
stages,  its  energy  ever  and  ever  integrally  present.  A  single  ray 
of  light  from  a  distant  star  falling  upon  the  eye  of  a  tyrant  in  by- 
gone times,  may  have  altered  the  course  of  his  life,  may  have 
changed  the  destiny  of  nations,  may  have  transformed  the  sur- 
face of  the  globe,  so  intricate,  so  inconceivably  complex  are  the 
processes  in  Nature.  In  no  way  can  we  get  such  an  overwhelm- 
ing idea  of  the  grandeur  of  Nature,  as  when  we  consider,  that  in 
accordance  with  the  law  of  the  conservation  of  energy,  throughout 
the  infinite,  the  forces  are  in  a  perfect  balance,-  and  hence  the 
energy  of  a  single  thought  may  determine  the  motion  of  a  Uni- 

*7"&#3i  tVVH/t. 

^^  ^  \£***^^svt*4&f  »  -^/-' 


HIGH  FREQUENCY  AND  HIGH  POTENTIAL  CURRENTS.      299 

verse.  It  is  not  necessary  that  every  individual,  not  even  that 
every  generation  or  many  generations,  should  have  the  physical 
instrument  of  sight,  in  order  to  be  able  to  form  images  and  to 
think,  that  is,  form  ideas  or  conceptions ;  but  sometime  or  other, 
during  the  process  of  evolution,  the  eye  certainly  must  have  ex- 
isted, else  thought,  as  we  understand  it,  would  be  impossible  ; 
else  conceptions,  like  spirit,  intellect,  mind,  call  it  as  you  may, 
could  not  exist.  It  is  conceivable,  that  in  some  other  world,  in 
some  other  beings,  the  eye  is  replaced  by  a  different  organ,  equally 
or  more  perfect,  but  these  beings  cannot  be  men. 

Nowr  what  prompts  us  all  to  voluntary  motions  and  actions  of 
any  kind  ?  Again  the  eye.  If  I  am  conscious  of  the  motion,  I 
must  have  an  idea  or  conception,  that  is,  an  image,  therefore  the 
eye.  If  I  am  not  precisely  conscious  of  the  motion,  it  is,  because 
the  images  are  vague  or  indistinct,  being  blurred  by  the  superim- 
position  of  many.  But  when  I  perform  the  motion,  does  the 
impulse  which  prompts  me  to  the  action  come  from  within  or  from 
without  ?  The  greatest  physicists  have  not  disdained  to  en- 
deavor to  answer  this  and  similar  questions  and  have  at  times 
abandoned  themselves  to  the  delights  of  pure  and  unrestrained 
thought.  Such  questions  are  generally  considered  not  to  belong 
to  the  realm  of  positive  physical  science,  but  will  before  long  be 
annexed  to  its  domain.  Helmholtz  has  probably  thought  more 
on  life  than  any  modern  scientist.  Lord  Kelvin  expressed  his 
belief  that  life's  process  is  electrical  and  that  there  is  a  force  in- 
herent to  the  organism  and  determining  its  motions.  Just  as 
much  as  I  am  convinced  of  any  physical  truth  I  am  convinced 
that  the  motive  impulse  must  come  from  the  outside.  For,  con- 
sider the  lowest  organism  we  know — and  there  are  probably 
many  lower  ones — an  aggregation  of  a  few  cells  only.'  If  it  is 
capable  of  voluntary  motion  it  can  perform  an  infinite  number 
of  motions,  all  definite  and  precise.  But  now  a  mechanism  con- 
sisting of  a  finite  number  of  parts  and  few  at  that,  cannot  per- 
form an  infinite  number  of  definite  motions,  hence  the  impulses 
which  govern  its  movements  must  come  from  the  environment. 
So,  the  atom,  the  ulterior  element  of  the  Universe's  structure,  is 
tossed  about  in  space,  eternally,  a  play  to  external  influences,  like 
a  boat  in  a  troubled  sea.  Were  it  to  stop  its  motion  it  would  die. 
Matter  at  rest,  if  such  a  thing  could  exist,  would  be  matter  dead. 
Death  of  matter !  Never  has  a  sentence  of  deeper  philosophical 
meaning  been  uttered.  This  is  the  way  in  which  Prof.  Dewar 


800  INVENTIONS  OF  NIKOLA  TESLA. 

forcibly  expresses  it  in  the  description  of  his  admirable  experi- 
ments, in  which  liquid  oxygen  is  handled  as  one  handles  water, 
and  air  at  ordinary  pressure  is  made  to  condense  and  even  to 
solidify  by  the  intense  cold.  Experiments,  which  serve  to  illus- 
trate, in  his  language,  the  last  feeble  manifestations  of  life,  the 
last  quiverings  of  matter  about  to  die.  But  human  eyes  shall 
not  witness  such  death.  There  is  no  death  of  matter,  for 
throughout  the  infinite  universe,  all  has  t3  move,  to  vibrate,  that 
is,  to  live. 

I  have  made  the  preceding  statements  at  the  peril  of  treading 
upon  metaphysical  ground,  in  my  desire  to  introduce  the  subject 
of  this  lecture  in  a  manner  not  altogether  uninteresting,  I  may 
hope,  to  an  audience  such  as  I  have  the  honor  to  address.  But 
now,  then,  returning  to  the  subject,  this  divine  organ  of  sight, 
this  indispensable  instrument  for  thought  and  all  intellectual  en- 
joyment, which  lays  open  to  us  the  marvels  of  this  universe, 
through  which  we  have  acquired  what  knowledge  we  possess,  and 
which  prompts  us  to,  and  controls,  all  our  physical  and  mental 
activity.  By  what  is  it  affected?  By  light !  What  is  light  ? 

We  have  witnessed  the  great  strides  which  have  been  made  in 
all  departments  of  science  in  recent  years.  So  great  have  been 
the  advances  that  we  cannot  refrain  from  asking  ourselves,  Is 
this  all  true,  or  is  it  but  a  dream  ?  Centuries  ago  men  have 
lived,  have  thought,  discovered,  invented,  and  have  believed  that 
they  were  soaring,  while  they  were  merely  proceeding  at  a  snail's 
pace.  So  we  too  may  be  mistaken.  But  taking  the  truth  of  the 
observed  events  as  one  of  the  implied  facts  of  science,  we  must 
rejoice  in  the  immense  progress  already  made  and  still  more  in  the 
anticipation  of  what  must  come,  judging  from  the  possibilities 
opened  up  by  modern  research.  There  is,  however,  an  advance 
which  we  have  been  witnessing,  which  must  be  particularly 
gratifying  to  every  lover  of  progress.  It  is  not  a  discovery,  or 
an  invention,  or  an  achievement  in  any  particular  direction.  It 
is  an  advance  in  all  directions  of  scientific  thought  and  experi- 
ment. I  mean  the  generalization  of  the  natural  forces  and  phe- 
nomena, the  looming  up  of  a  certain  broad  idea  on  the  scientific 
horizon.  It  is  this  idea  which  has,  however,  long  ago  taken  pos- 
session of  the  most  advanced  minds,  to  which  I  desire  to  call  your 
attention,  and  which  I  intend  to  illustrate  in  a  general  way,  in 
these  experiments,  as  the  first  step  in  answering  the  question 
"What  is  light?"  and  to  realize  the  modern  meaning  of  this 
word. 


HIGH  FREQUENCY  AND  HIGH  POTENTIAL  CURRENTS.     301 

It  is  beyond  the  scope  of  my  lecture  to  dwell  upon  the  subject 
of  light  in  general,  my  object  being  merely  to  bring  presently  to 
your  notice  a  certain  class  of  light  effects  and  a  number  of  phe- 
nomena observed  in  pursuing  the  study  of  these  effects.  But  to 
be  consistent  in  my  remarks  it  is  necessary  to  state  that,  according 
to  that  idea,  now  accepted  by  the  majority  of  scientific  men  as  a 
positive  result  of  theoretical  and  experimental  investigation,  the 
various  forms  or  manifestations  of  energy  which  were  generally 
designated  as  "electric"  or  more  precisely  "electromagnetic  "  are 
energy  manifestations  of  the  same  nature  as  those  of  radiant 
heat  and  light.  Therefore  the  phenomena  of  light  and  heat  and 
others  besides  these,  may  be  called  electrical  phenomena.  Thus 
electrical  science  has  become  the  mother  science  of  all  and  its 
study  has  become  all  important.  The  day  when  we  shall  know 
exactly  what  "electricity"  is,  will  chronicle  an  event  probably 
greater,  more  important  than  any  other  recorded  in  the  history 
of  the  human  race.  The  time  will  come  when  the  comfort,  the 
very  existence,  perhaps,  of  man  will  depend  upon  that  wonderful 
agent.  For  our  existence  and  comfort  we  require  heat,  light 
and  mechanical  power.  How  do  we  now  get  all  these?  We  get 
them  from  fuel,  we  get  them  by  consuming  material.  What 
will  man  do  when  the  forests  disappear,  when  the  coal  fields  are 
exhausted  ?  Only  one  thing,  according  to  our  present  knowledge 
will  remain ;  that  is,  to  transmit  power  at  great  distances.  Men 
will  go  to  the  waterfalls,  to  the  tides,  which  are  the  stores  of  an 
infinitesimal  part  of  Nature's  immeasurable  energy.  There  will 
they  harness  the  energy  and  transmit  the  same  to  their  settle- 
ments, to  warm  their  homes  by,  to  give  them  light,  and  to  keep 
their  ooedient  slaves,  the  machines,  toiling.  But  how  will  they 
transmit  this  energy  if  not  by  electricity  ?  Judge  then,  if  the 
comfort,  nay,  the  very  existence,  of  man  will  not  depend  on  elec- 
tricity. I  am  aware  that  this  view  is  not  that  of  a  practical 
engineer,  but  neither  is  it  that  of  an  illusionist,  for  it  is  certain, 
that  power  transmission,  which  at  present  is  merely  a  stimulus  to 
enterprise,  will  some  day  be  a  dire  necessity. 

It  is  more  important  for  the  student,  who  takes  up  the  study 
of  light  phenomena,  to  make  himself  thoroughly  acquainted  with 
certain  modern  views,  than  to  peruse  entire  books  on  the  subject 
of  light  itself,  as  disconnected  from  these  views.  Were  I  there- 
fore to  make  these  demonstrations  before  students  seeking 
information — and  for  the  sake  of  the  few  of  those  who  may  be 


802  INVENTIONS  OF  NIKOLA  TESLA. 

present,  give  me  leave  to  so  assume — it  would  be  my  principal 
endeavor  to  impress  these  views  upon  their  minds  in  this  series  of 
experiments. 

It  might  be  sufficient  for  tins  purpose  to  perform  a  simple  and 
well-known  experiment.  I  might  take  a  familiar  appliance,  a 
L3yden  jar,  charge  it  from  a  frictional  machine,  and  then  dis- 
charge it.  In  explaining  to  you  its  permanent  state  when  charged, 
and  its  transitory  condition  when  discharging,  calling  your  atten- 
tion to  the  forces  which  enter  into  play  and  to  the  various  phen- 
omena they  produce,  and  pointing  out  the  relation  of  the  forces 
and  phenomena,  I  might  fully  succeed  in  illustrating  that  modern 
idea.  Xo  doubt,  to  the  thinker,  this  simple  experiment  would 
appeal  as  much  as  the  most  magnificent  display.  But  this  is  to 
be  an  experimental  demonstration,  and  one  which  should  possess, 
besides  instructive,  also  entertaining  features  and  as  such,  a  simple 
experiment,  such  as  the  one  cited,  would  not  go  very  far  towards 
the  attainment  of  the  lecturer's  aim.  I  must  therefore  choose 
another  way  of  illustrating,  more  spectacular  certainly,  but  per- 
haps also  more  instructive.  Instead  of  the  frictional  machine  and 
Leyden  jar,  I  shall  avail  myself  in  these  experiments,  of  an  induc- 
tion coil  of  peculiar  properties,  which  was  described  in  detail  by  me 
in  a  lecture  before  the  London  Institution  of  Electrical  Engineers, 
in  Feb.,  1892.  This  induction  coil  is  capable  of  yielding  currents  of 
enormous  potential  differences,  alternating  with  extreme  rapidity. 
"With  this  apparatus  I  shall  endeavor  to  show  you  three  distinct 
classes  of  effects,  or  phenomena,  and  it  is  my  desire  that  each 
experiment,  while  serving  for  the  purposes  of  illustration,  should 
at  the  same  time  teach  us  some  novel  truth,  or  show  us  some 
novel  aspect  of  this  fascinating  science.  But  before  doing  this,  it 
seems  proper  and  useful  to  dwell  upon  the  apparatus  employed, 
and  method  of  obtaining  the  high  potentials  and  high-frequency 
currents  which  are  made  use  of  in  these  experiments. 

ON  THE  APPARATUS  AND  METHOD  OF  CONVERSION. 

These  high-frequency  currents  are  obtained  in  a  peculiar  man- 
ner. The  method  employed  was  advanced  by  me  about  two 
years  ago  in  an  experimental  lecture  before  the  American  Insti- 
tute of  Electrical  Engineers.  A  number  of  ways,  as  practiced  in 
the  laboratory,  of  obtaining  these  currents  either  from  continuous 
or  low  frequency  alternating  currents,  is  diagramatically  indicated 
in  Fig.  165,  which  will  be  later  described  in  detail.  The  general 


HIGH  FREQUENCY  AND  HIGH  POTENTIAL  CURRENTS. 


304  INVENTIONS  OF  NIKOLA  TESLA. 

plan  is  to  charge  condensers,  from  a  direct  or  alternate-current 
source,  preferably  of  high-tension,  and  to  discharge  them 
disruptively  while  observing  well-known  conditions  neces- 
sary to  maintain  the  oscillations  of  the  current.  In  view  of  the 
general  interest  taken  in  high-frequency  currents  and  effects  pro- 
ducible by  them,  it  seems  to  me  advisable  to  dwell  at  some  length 
upon  this  method  of  conversion.  In  order  to  give  you  a  clear 
idea  of  the  action,  I  will  suppose  that  a  continuous-current  gen- 
erator is  employed,  which  is  often  very  convenient.  It  is  desirable 
that  the  generator  should  possess  such  high  tension  as  to  be  able 
to  break  through  a  small  air  space.  If  this  is  not  the  case,  then 
auxiliary  means  have  to  be  resorted  to,  some  of  which  will  be  in- 
dicated subsequently.  When  the  condensers  are  charged  to  a 
certain  potential,  the  air,  or  insulating  space,  gives  way  and  a  dis- 
ruptive discharge  occurs.  There  is  then  a  sudden  rush  of  current 
and  generally  a  large  portion  of  accumulated  electrical  energy 
spends  itself.  The  condensers  are  thereupon  quickly  charged  and 
the  same  process  is  repeated  in  more  or  less  rapid  succession. 
To  produce  such  sadden  rushes  of  current  it  is  necessary  to  ob- 
serve certain  conditions.  If  the  rate  at  which  the  condensers  are 
disci mrged  is  the  same  as  that  at  which  they  are  charged,  then, 
clearly,  in  the  assumed  case  the  condensers  do  not  come  into 
play.  If  the  rate  of  discharge  be  smaller  than  the  rate  of  charg- 
ing, then,  again,  the  condensers  cannot  play  an  important  part. 
But  if,  on  the  contrary,  the  rate  of  discharging  is  greater  than 
that  of  charging,  then  a  succession  of  rushes  of  current  is  ob- 
tained. It  is  evident  that,  if  the  rate  at  which  the  energy  is 
dissipated  by  the  discharge  is  very  much  greater  than  the  rate  of 
supply  to  the  condensers,  the  sudden  rushes  will  be  compara- 
tively few,  with  long-time  intervals  between.  This  alwavs  occurs 
when  a  condenser  of  considerable  capacity  is  charged  by  means 
of  a  comparatively  small  machine.  If  the  rates  of  supply  and 
dissipation  are  not  widely  different,  then  the  rushes  of  current 
will  be  in  quicker  succession,  and  this  the  more,  the  more  nearly 
equal  both  the  rates  are,  until  limitations  incident  to  eacli  case 
and  depending  upon  a  number  of  causes  are  reached.  Thus  we 
are  able  to  obtain  from  a  continuous-current  generator  as  rapid  a 
succession  of  discharges  as  we  like.  Of  course,  the  higher  the 
tension  of  the  generator,  the  smaller  need  be  the  capacity  of  the 
condensers,  and  for  this  reason,  principally,  it  is  of  advantage  to 
employ  a  generator  of  very  high  tension.  Besides,  such  a  gener- 
ator permits  the  attaining  of  greater  rates  of  vibration. 


1UG11  FREQUENCY  AND  HIGH  POTENTIAL  CURRENTS.     305 

The  rushes  of  current  may  be  of  the  same  direction  under  the 
conditions  before  assumed,  but  most  generally  there  is  an  oscilla- 
tion superimposed  upon  the  fundamental  vibration  of  the  current. 
When  the  conditions  are  so  determined  that  there  are  no  oscilla- 
tions, the  current  impulses  are  unidirectional  and  thus  a  means  is 
provided  of  transforming  a  continuous  current  of  high  tension, 
into  a  direct  current  of  lower  tension,  which  I  think  may  find 
employment  in  the  arts. 

This  method  of  conversion  is  exceedingly  interesting  and  I 
was  much  impressed  by  its  beauty  when  I  first  conceived  it.  It  is 
ideal  in  certain  respects.  It  involves  the  employment  of  no  me- 
chanical devices  of  any  kind,  and  it  allows  of  obtaining  currents 
of  any  desired  frequency  from  an  ordinary  circuit,  direct  or  al- 
ternating. The  frequency  of  the  fundamental  discharges  depend- 
ing on  the  relative  rates  of  supply  and  dissipation  can  be  readily 
varied  within  wide  limits,  by  simple  adjustments  of  these  quanti- 
ties, and  the  frequency  of  the  superimposed  vibration  by  the 
determination  of  the  capacity,  self-induction  and  resistance  of  the 
circuit.  The  potential  of  the  currents,  again,  may  be  raised  as 
high  as  any  insulation  is  capable  of  withstanding  safely  by  com- 
bining capacity  and  self-induction  or  by  induction  in  a  secondary, 
which  need  have  but  comparatively  few  turns. 

As  the  conditions  are  often  such  that  the  intermittence  or  os- 
cillation does  not  readily  establish  itself,  especially  when  a  direct 
current  source  is  employed,  it  is  of  advantage  to  associate  an  in- 
terrupter with  the  arc,  as  I  have,  some  time  ago,  indicated  the 
use  of  an  air-blast  or  magnet,  or  other  such  device  readily  at 
hand.  The  magnet  is  employed  with  special  advantage  in  the 
conversion  of  direct  currents,  as  it  is  then  very  effective.  If  the 
primary  source  is  an  alternate  current  generator,  it  is  desirable, 
as  I  have  stated  on  another  occasion,  that  the  frequency  should 
be  low,  and  that  the  current  forming  the  arc  be  large,  in  order 
to  render  the  magnet  more  effective. 

A  form  of  such  discharger  with  a  magnet  which  has  been 
found  convenient,  and  adopted  after  some  trials,  in  the  conversion 
of  direct  currents  particularly,  is  illustrated  in  Fig.  166.  N  s  are 
the  pole  pieces  of  a  very  strong  magnet  which  is  excited  by  a  coil 
c.  The  pole  pieces  are  slotted  for  adjustment  and  can  be  fastened 
in  any  position  by  screws  s  s^  The  discharge  rods  d  dt1  thinned 
down  on  the  ends  in  order  to  allow  a  closer  approach  of  the  mag- 
netic pole  pieces,  pass  through  the  columns  of  brass  b  ^  and  are 
fastened  in  position  by  screws  #2  $2-  Springs  r  rt  and  collars  c  c± 


306  INVENTIONS  OF  NIKOLA  TESLA. 

are  slipped  on  the  rods,  the  latter  serving  to  set  the  points  of  the 
rods  at  [a  certain  suitable  distance  by  means  of  screws  #3  ss,  and 
the  former  to  draw  the  points  apart.  When  it  is  desired  to  start 
the  arc,  one  of  the  large  rubber  handles  h  Ji±  is  tapped  quickly 
with  the  [hand,  whereby  the  points  of  the  rods  are  brought  in 
contact  but  are  instantly  separated  by  the  springs  r  r^  Such  an 
arrangements-has  been  found  to  be  often  necessary,  namely  in 
cases  when  the  E.  M.  r.  was  not  large  enough  to  cause  the  discharge 
to  break  through  the  gap,  and  also  when  it  was  desirable  to  avoid 
short  circuiting  of  the  generator  by  the  metallic  contact  of  the 
rods.  The  rapidity  of  the  interruptions  of  the  current  with  a 
magnet  depends  on  the  intensity  of  the  magnetic  field  and  on  the 


FIG.  ICG. 

potential  difference  at  the  end  of  the  arc.  The  interruptions  are 
generally  in  such  quick  succession  as  to  produce  a  musical  sound. 
Years  ago  it  was  observed  that  when  a  powerful  induction  coil 
is  discharged  between  the  poles  of  a  strong  magnet,  the  discharge 
produces  a  loud  noise,  not  unlike  a  small  pistol  shot,  It  was 
vaguely  stated  that  the  spark  was  intensified  by  the  presence  of 
the  magnetic  field.  It  is  now  clear  that  the  discharge  current, 
flowing  for  some  time,  was  interrupted  a  great  number  of  times 
by  the  magnet,  thus  producing  the  sound.  The  phenomenon  is 
especially  marked  when  the  field  circuit  of  a  large  magnet  or 
dynamo  is  broken  in  a  powerful  magnetic  field. 


HIGH  FREQUENCE:  AND  HIGH  POTENTIAL  CURRENTS.    307 

When  the  current  through  the  gap  is  comparatively  large,  it  is 
of  advantage  to  slip  on  the  points  of  the  discharge  rods  pieces  of 
very  hard  carbon  and  let  the  arc  play  between  the  carbon  pieces. 
This  preserves  the  rods,  and  besides  has  the  advantage  of  keep- 
ing the  air  space  hotter,  as  the  heat  is  not  conducted  away  as 
quickly  through  the  carbons,  and  the  result  is  that  a  smaller 
E.  M.  F.  in  the  arc  gap  is  required  to  maintain  a  succession  of 
discharges. 

Another  form  of  discharger,  which  may  be  employed  with  ad- 
vantage in  some  cases,  is  illustrated  in  Fig.  167.  In  this  form 
the  discharge  rods  d  d^  pass  through  perforations  in  a  wooden 


FIG.  107. 


box  B,  which  is  thickly  coated  with  mica  on  the  inside,  as  indi- 
cated by  the  heavy  lines.  The  perforations  are  provided  with 
mica  tubes  m  m^  of  some  thickness,  which  are  preferably  not  in 
contact  with  the  rods  d  d{.  The  box  has  a  cover  c  which  is  a 
little  larger  and  descends  on  the  outside  of  the  box.  The  spark 
gap  is  warmed  by  a  small  lamp  I  contained  in  the  box.  A  plate 
p  above  the  lamp  allows  the  draught  to  pass  only  through  the 
chimney  <?  of  the  lamp,  the  air  entering  through  holes  o  o  in  or 
near  the  bottom  of  the  box  and  following  the  path  indicated  by 
the  arrows.  When  the  discharger  is  in  operation,  the  door  of  the 
box  is  closed  so  that  the  light  of  the  arc  is  not  visible  outside. 


308  INVENTIONS  OF  NIKOLA  TESLA. 

It  is  desirable  to  exclude  the  light  as  perfectly  as  possible,  as  it 
interferes  with  some  experiments.  This  form  of  discharger  is  sim- 
ple and  very  effective  when  properly  manipulated.  The  air 
being  warmed  to  a  certain  temperature,  has  its  insulating  power 
impaired ;  it  becomes  dielectrically  weak,  as  it  were,  and  the  con- 
sequence is  that  the  arc  can  be  established  at  much  greater  dis- 
tance. The  arc  should,  of  course,  be  sufficiently  insulating  to 
allow  the  discharge  to  pass  through  the  gap  disruptively.  The 
arc  formed  under  such  conditions,  when  long,  may  be  made  ex- 
tremely sensitive,  and  the  weak  draught  through  the  lamp 
chimney  c  is  quite  sufficient  to  produce  rapid  interruptions.  The 
adjustment  is  made  by  regulating  the  temperature  and  velocity 
of  the  draught.  Instead  of  using  the  lamp,  it  answers  the  pur- 
pose to  provide  for  a  draught  of  warm  air  in  other  ways.  A 
very  simple  way  which  has  been  practiced  is  to  enclose  the  arc 
in  a  long  vertical  tube,  with  plates  on  the  top  and  bottom  for 
regulating  the  temperature  and  velocity  of  the  air  current. 
Some  provision  had  to  be  made  for  deadening  the  sound. 

The  air  may  be  rendered  dielectrically  weak  also  by  rarefac- 
tion. Dischargers  of  this  kind  have  likewise  been  used  by  me 
in  connection  with  a  magnet.  A  large  tube  is  for  this  purpose 
provided  with  heavy  electrodes  of  carbon  or  metal,  between 
which  the  discharge  is  made  to  pass,  the  tube  being  placed  in  a 
powerful  magnetic  field.  The  exhaustion  of  the  tube  is  carried 
to  a  point  at  which  the  discharge  breaks  through  easily,  but  the 
pressure  should  be  more  than  Y5  millimetres,  at  which  the  ordi. 
nary  thread  discharge  occurs.  In  another  form  of  discharger, 
combining  the  features  before  mentioned,  the  discharge  was 
made  to  pass  between  two  adjustable  magnetic  pole  pieces,  the 
space  between  them  being  kept  at  an  elevated  temperature. 

It  should  be  remarked  here  that  when  such,  or  interrupting 
devices  of  any  kind,  are  used  and  the  currents  are  passed  through 
the  primary  of  a  disruptive  discharge  coil,  it  is  not,  as  a  rule,  of 
advantage  to  produce  a  number  of  interruptions  of  the  current 
per  second  greater  than  the  natural  frequency  of  vibration  of  the 
dynamo  supply  circuit,  which  is  ordinarily  small.  It  should  also 
ba  pointed  out  here,  that  while  the  devices  mentioned  in  connec- 
tion with  the  disruptive  discharge  are  advantageous  under  cer- 
tain conditions,  they  may  be  sometimes  a  source  of  trouble,  as 
they  produce  intermittences  and  other  irregularities  in  the  vibra- 
tion which  it  would  be  very  desirable  to  overcome. 


IIIGII  FREQUENCY  AND  IIIGI1  POTENTIAL  CURRENTS.      309 

There  is,  I  regret  to  say,  in  this  beautiful  method  of  conversion 
a  defect,  which  fortunately  is  not  vital,  and  which  I  have  been 
gradually  overcoming.  I  will  best  call  attention  to  this  defect 
and  indicate  a  fruitful  line  of  work,  by  comparing  the  electrical 
process  with  its  mechanical  analogue.  The  process  may  be  illus- 
trated in  this  manner.  Imagine  a  tank  with  a  wide  opening  at 
the  bottom,  which  is  kept  closed  by  spring  pressure,  but  so  that 
it  snaps  off  suddenly  when  the  liquid  in  the  tank  has  reached  a 
certain  height.  Let  the  fluid  be  supplied  to  the  tank  by  means 
of  a  pipe  feeding  at  a  certain  rate.  When  the  critical  height  of 
the  liquid  is  reached,  the  spring  gives  way  and  the  bottom  of  the 
tank  drops  out.  Instantly  the  liquid  falls  through  the  wide  open- 
ing, and  the  spring,  reasserting  itself,  closes  the  bottom  again. 
The  tank  is  now  filled,  and  after  a  certain  time  interval  the  same 
process  is  repeated.  It  is  clear,  that  if  the  pipe  feeds  the  fluid 
quicker  than  the  bottom  outlet  is  capable  of  letting  it  pass 
through,  the  bottom  will  remain  off.  and  the  tank  will  still  overflow. 
If  the  rates  of  supply  are  exactly  equal,  then  the  bottom  lid  will 
remain  partially  open  and  no  vibration  of  the  same"  and  of  the 
liquid  column  will  generally  occur,  though  it  might,  if  started  by 
some  means.  But  if  the  inlet  pipe  does  not  feed  the  fluid  fast 
enough  for  the  outlet,  then  there  will  be  always  vibration. 
Again,  in  such  case,  each  time  the  bottom  flaps  up  or  down,  the 
spring  and  the  liquid  column,  if  the  pliability  of  the  spring  and 
the  inertia  of  the  moving  parts  are  properly  chosen,  will  perform 
independent  vibrations.  In  this  analogue  the  fluid  may  be  lik- 
ened to  electricity  or  electrical  energy,  the  tank  to  the  condenser, 
the  spring  to  the  dielectric,  and  the  pipe  to  the  conductor  through 
which  electricity  is  supplied  to  the  condenser.  To  make  this 
analogy  quite  complete  it  is  necessary  to  make  the  assumption, 
that  the  bottom,  each  time  it  gives  way,  is  knocked  violently 
against  a  non-elastic  stop,  this  impact  involving  some  loss  of  en- 
ergy ;  and  that,  besides,  some  dissipation  of  energy  results  due  to 
frictional  losses.  In  the  preceding  analogue  the  liquid  is  sup- 
posed to  be  under  a  steady  pressure.  If  the  presence  of  the  fluid 
be  assumed  to  vary  rhythmically,  this  may  be  taken  as  corres- 
ponding to  the  case  of  an  alternating  current.  The  process  is 
then  not  quite  as  simple  to  consider,  but  the  action  is  the  same  in 
principle. 

It  is  desirable,  in  order  to  maintain  the  vibration  economically, 
to  reduce  the  impact  and  frictional  losses  as  much  as  possible. 


310  INVENTIONS  OF  NIKOLA  TKSLA. 

As  regards  the  latter,  which  in  the  electrical  analogue  correspond 
to  the  losses  due  to  the  resistance  of  the  circuits,  it  is  impossible 
to  obviate  them  entirely,  but  they  can  be  reduced  to  a  minimum 
by  a  proper  selection  of  the  dimensions  of  the  circuits  and  by  the 
the  employment  of  thin  conductors  in  the  form  of  strands.  But 
the  loss  of  energy  caused  by  the  first  breaking  through  of  the 
dielectric — which  in  the  above  example  corresponds  to  the  violent 
knock  of  the  bottom  against  the  inelastic  stop — would  be  more  im- 
portant to  overcome.  At  the  moment  of  the  breaking  through, 
the  air  space  has  a  very  high  resistance,  which  is  probably  re- 
duced to  a  very  small  value  when  the  current  has  reached  some 
strength,  and  the  space  is  brought  to  a  high  temperature.  It 
would  materially  diminish  the  loss  of  energy  if  the  space  were 
always  kept  at  an  extremely  high  temperature,  but  then  there 
would  be  no  disruptive  break.  By  warming  the  space  moder- 
ately by  means  of  a  lamp  or  otherwise,  the  economy  as  far  as  the 
arc  is  concerned  is  sensibly  increased.  But  the  magnet  or  other 
interrupting  device  does  not  diminish  the  loss  in  the  arc.  Like- 
wise, a  jet  of  air  only  facilitates  the  carrying  off  of  the  energy. 
Air,  or  a  gas  in  general,  behaves  curiously  in  this  respect.  When 
two  bodies  charged  to  a  very  high  potential,  discharge  disrupt- 
ively  through  an  air  space,  any  amount  of  energy  may  be  carried 
off  by  the  air.  This  energy  is  evidently  dissipated  by  bodily 
carriers,  in  impact  and  collisional  losses  of  the  molecules.  The 
exchange  of  the  molecules  in  the  space  occurs  with  inconceivable 
rapidity.  A  powerful  discharge  taking  place  between  two  elec- 
trodes, they  may  remain  entirely  cool,  and  yet  the  loss  in  the 
air  may  represent  any  amount  of  energy.  It  is  perfectly  prac- 
ticable, with  very  great  potential  differences  in  the  gap,  to  dissi- 
pate several  horse-power  in  the  arc  of  the  discharge  without  even 
noticing  a  small  increase  in  the  temperature  of  the  electrodes. 
All  the  frictional  losses  occur  then  practically  in  the  air.  If  the 
exchange  of  the  air  molecules  is  prevented,  as  by  enclosing  the  air 
hermetically,  the  gas  inside  of  the  vessel  is  brought  quickly  to  a 
high  temperature,  even  with  a  very  small  discharge.  It  is  diffi- 
cult to  estimate  how  much  of  the  energy  is  lost  in  sound  waves, 
audible  or  not,  in  a  powerful  discharge.  When  the  currents 
through  the  gap  are  large,  the  electrodes  may  become  rapidly 
heated,  but  this  is  not  a  reliable  measure  of  the  energy  wasted  in 
the  arc,  as  the  loss  through  the  gap  itself  may  be  comparatively 
small.  The  air  or  a  gas  in  general  is,  'at  ordinary  pressure  at  least, 


HIGH  FREQUENCY  AND  HIGH  POTENTIAL  CURRENTS.     311 

clearly  not  the  best  medium  through  which  a  disruptive  dis- 
charge should  occur.  Air  or  other  gas  under  great  pressure  is  of 
course  a  much  more  suitable  medium  for  the  discharge  gap.  I 
have  carried  on  long-continued  experiments  in  this  direction,  un- 
fortunately less  practicable  on  account  of  the  difficulties  and  ex- 
pense in  getting  air  under  great  pressure.  But  even  if  the 
medium  in  the  discharge  space  is  solid  or  liquid,  still  the  same 
losses  take  place,  though  they  are  generally  smaller,  for  jusfr  as 
soon  as  the  arc  is  established,  the  solid  or  liquid  is  volatilized. 
Indeed,  there  is  no  body  known  which  would  not  be  disintegrated 
by  the  arc,  and  it  is  an  open  question  among  scientific  men, 
whether  an  arc  discharge  could  occur  at  all  in  the  air  itself  with- 
out the  particles  of  the  electrodes  being  torn  off.  When  the 
current  through  the  gap  is  very  small  and  the  arc  very  long,  I 
believe  that  a  relatively  considerable  amount  of  heat  is  taken  up 
in  the  disintegration  of  the  electrodes,  which  partially  on  this  ac- 
count may  remain  quite  cold. 

The  ideal  medium  for  a  discharge  gap  should  only  crack,  and 
the  ideal  electrode  should  be  of  some  material  which  cannot  be 
disintegrated.  With  small  currents  through  the  gap  it  is  best  to 
employ  aluminum,  but  not  when  the  currents  are  large.  The  dis- 
ruptive break  in  the  air,  or  more  or  less  in  any  ordinary  medium, 
is  not  of  the  nature  of  a  crack,  but  it  is  rather  comparable  to  the 
piercing  of  innumerable  bullets  through  a  mass  offering  great 
frictional  resistances  to  the  motion  of  the  bullets,  this  involving 
considerable  loss  of  energy.  A  medium  which  would  merely 
crack  when  strained  electrostatically — and  this  possibly  might  be 
the  case  with  a  perfect  vacuum,  that  is,  pure  ether — would  involve 
a  very  small  loss  in  the  gap,  so  small  as  to  be  entirely  negligible, 
at  least  theoretically,  because  a  crack  may  be  produced  by  an 
infinitely  small  displacement.  In  exhausting  an  oblong  bulb 
provided  with  two  aluminum  terminals,  with  the  greatest  care,  I 
have  succeeded  in  producing  such  a  vacuum  that  the  secondary 
discharge  of  a  disruptive  discharge  coil  would  break  disrup- 
tively  through  the  bulb  in  the  form  of  fine  spark  streams.  The 
curious  point  was  that  the  discharge  would  completely  ignore  the 
terminals  and  start  far  behind  the  two  aluminum  plates  which 
served  as  electrodes.  This  extraordinary  high  vacuum  could  only 
be  maintained  for  a  very  short  while.  To  return  to  the  ideal 
medium,  think,  for  the  sake  of  illustration,  of  a  piece  of  glass  or 
similar  body  clamped  in  a  vice,  and  the  latter  tightened  more  and 


312  INVENTIONS  OF  NIKOLA  TESLA, 

more.  At  a  certain  point  a  minute  increase  of  the  pressure  will 
cause  the  glass  to  crack.  The  loss  of  energy  involved  in  splitting 
the  glass  may  be  practically  nothing,  for  though  the  force  is  great, 
the  displacement  need  be  but  extremely  smalL  Now  imagine 
that  the  glass  would  possess  the  property  of  closing  again  per- 
fectly the  crack  upon  a  minute  diminution  of  the  pressure. 
This  is  the  way  the  dielectric  in  the  discharge  space  should 
behave.  But  inasmuch  as  there  would  be  always  some  loss  in  the 
gap,  the  medium,  which  should  be  continuous,  should  exchange 
through  the  gap  at  a  rapid  rate.  In  the  preceding  example,  the 
glass  being  perfectly  closed,  it  would  mean  that  the  dielectric  in 
the  discharge  space  possesses  a  great  insulating  power ;  the  glass 
being  cracked,  it  would  signify  that  the  medium  in  the  space  is 
a  good  conductor.  The  dielectric  should  vary  enormously  in 
resistance  by  minute  variations  of  the  E.  M.  F.  across  the 
discharge  space.  This  condition  is  attained,  but  in  an  extremely 
imperfect  manner,  by  warming  the  air  space  to  a  certain 
critical  temperature,  dependent  on  the  E.  M.  F.  across  the  gap, 
or  by  otherwise  impairing  the  insulating  power  of  the  air.  But 
as  a  matter  of  fact  the  air  does  never  break  down  disruptively, 
if  this  term  be  rigorously  interpreted,  for  before  the  sudden 
rush  of  the  current  occurs,  there  is  always  a  weak  current 
preceding  it,  which  rises  first  gradually  and  then  with  compara- 
tive suddenness.  That  is  the  reason  why  the  rate  of  change  is 
very  much  greater  when  glass,  for  instance,  is  broken  through, 
than  when  the  break  takes  place  through  an  air  space  of  equiva- 
lent dielectric  strength.  As  a  medium  for  the  discharge  space,  a 
solid,  or  even  a  liquid,  would  be  preferable  therefor.  It  is  some- 
what difficult  to  conceive  of  a  solid  body  which  would  possess  the 
property  of  closing  instantly  after  it  has  been  cracked.  But  a 
liquid,  especially  under  great  pressure,  behaves  practically  like  a 
solid,  while  it  possesses  the  property  of  closing  the  crack.  Hence 
it  was  thought  that  a  liquid  insulator  might  be  more  suitable  as  a 
dielectric  than  air.  Following  out  this  idea,  a  number  of  different 
forms  of  dischargers  in  which  a  variety  of  such  insulators,  some- 
times under  great  pressure,  were  employed,  have  been  experi- 
mented upon.  It  is  thought  sufficient  to  dwell  in  a  few  words 
upon  one  of  the  forms  experimented  upon.  One  of  these  dis- 
chargers is  illustrated  in  Figs.  168«  and  168 b. 

A  hollow  metal  pulley  P  (Fig.  16  8«),  was  fastened  upon  an  ar- 
bor #,  which  by  suitable  means  was  rotated  at  a  considerable 


HIQII  FREQUENCY  AND  HIOII  POTENTIAL  CURRENTS.      313 

speed.  On  the  inside  of  the  pulley,  but  disconnected  from  the 
same,  was  supported  a  thin  disc  h  (which  is  shown  thick  for  the 
sake  of  clearness),  of  hard  rubber  in  which  there  were  embedded 
two  metal  segments  s  s  with  metallic  extensions  e  e  into  which 
were  screwed  conducting  terminals  t  t  covered  with  thick  tubes 
of  hard  rubber  1 1.  The  rubber  disc  h  with  its  metallic  segments 
s  ,9,  was  finished  in  a  lathe,  and  its  entire  surface  highly  polished 
so  as  to  offer  the  smallest  possible  frictional  resistance  to  the  mo- 
tion through  a  fluid.  In  the  hollow  of  the  pulley  an  insulating 
liquid  such  as  a  thin  oil  was  poured  so  as  to  reach  very  nearly  to 
the  opening  left  in  the  flange/,  which  was  screwed  tightly  on  the 
front  side  of  the  pulley.  The  terminals  t  t,  were  connected  to  the 
opposite  coatings  of  a  battery  of  condensers  so  that  the  discharge 
occurred  through  the  liquid.  When  the  pulley  was  rotated,  the 
liquid  was  forced  against  the  rim  of  the  pulley  and  considerable 
fluid  pressure  resulted.  In  this  simple  way  the  discharge  gap 


FIG.  168a. 


FIG.  168b. 


was  filled  with  a  medium  which  behaved  practically  like  a  solid, 
which  possessed  the  quality  of  closing  instantly  upon  the  occur- 
rence of  the  break,  and  which  moreover  was  circulating  through 
the  gap  at  a  rapid  rate.  Very  powerful  effects  were  produced  by 
discharges  of  this  kind  with  liquid  interrupters,  of  which  a  num- 
ber of  different  forms  were  made.  It  was  found  that,  as  ex- 
pected, a  longer  spark  for  a  given  length  of  wire  was  obtainable 
in  this  way  than  by  using  air  as  an  interrupting  device.  Gener- 
ally the  speed,  and  therefore  also  the  fluid  pressure,  was  limited 
by  reason  of  the  fluid  friction,  in  the  form  of  discharger  described, 
but  the  practically  obtainable  speed  was  more  than  sufficient  to 
produce  a  number  of  breaks  suitable  for  the  circuits  ordinarily 
used.  In  such  instances  the  metal  pulley  P  was  provided  with  a 
few  projections  inwardly,  and  a  definite  number  of  breaks  was 
then  produced  which  could  be  computed  from  the  speed  of 


314  1A'  VJHNTION8  OF  NIKOLA  TESLA. 

rotation  of  the  pulley.  Experiments  were  also  carried  on  with 
liquids  of  different  insulating  power  with  the  view  of  reducing 
the  loss  in  the  arc.  When  an  insulating  liquid  is  moderately 
warmed,  the  loss  in  the  arc  is  diminished. 

A  point  of  some  importance  was  noted  in  experiments  with 
various  discharges  of  this  kind.  It  was  found,  for  instance,  that 
whereas  the  conditions  maintained  in  these  forms  were  favorable 
for  the  production  of  a  great  spark  length,  the  current  so  ob- 
tained was  not  best  suited  to  the  production  of  light  effects.  Ex- 
perience undoubtedly  has  shown,  that  for  such  purposes  a  har- 
monic rise  and  fall  of  the  potential  is  preferable.  Be  it  that  a 
solid  is  rendered  incandescent,  or  phosphorescent,  or  be  it  that  en- 
ergy is  transmitted  by  condenser  coating  through  the  glass,  it  is 
quite  certain  that  a  harmonically  rising  and  falling  potential  pro- 
duces less  destructive  action,  and  that  the  vacuum  is  more  per- 
manently maintained.  This  would  be  easily  explained  if  it  were 
ascertained  that  the  process  going  on  in  an  exhausted  vessel  is  of 
an  electrolytic  nature. 

In  the  diagrammatical  sketch,  Fig.  165,  which  has  been  already 
referred  to,  the  cases  which  are  most  likely  to  be  met  with  in 
practice  are  illustrated.  One  has  at  his  disposal  either  direct  or 
alternating  currents  from  a  supply  station.  It  is  convenient  for 
an  experimenter  in  an  isolated  laboratory  to  employ  a  machine  G, 
such  as  illustrated,  capable  of  giving  both  kinds  of  currents.  In 
such  case  it  is  also  preferable  to  use  a  machine  with  multiple 
circuits,  as  in  many  experiments  it  is  useful  and  convenient  to 
have  at  one's  disposal  currents  of  different  phases.  In  the 
sketch,  D  represents  the  direct  and  A  the  alternating  circuit.  In 
each  of  these,  three  branch  circuits  are  shown,  all  of  which  are 
provided  with  double  line  switches  s  s  s  s  s  s.  Consider  first  the 
direct  current  conversion ;  m  represents  the  simplest  case.  If 
the  E.  M.  F.  of  the  generator  is  sufficient  to  break  through  a  small 
air  space,  at  least  when  the  latter  is  warmed  or  otherwise  rend- 
ered poorly  insulating,  there  is  no  difficulty  in  maintaining  a 
vibration  with  fair  economy  by  judicious  adjustment  of  the 
capacity,  self-induction  and  resistance  of  the  circuit  L  containing 
the  devices  II  m.  The  magnet  N,  s,  can  be  in  this  case  advan- 
tageously combined  with  the  air  space.  The  discharger  d  d  with 
the  magnet  may  be  placed  either  way,  as  indicated  by  the  full  or 
by  the  dotted  lines.  The  circuit  la  with  the  connections  and  de- 
vices is  supposed  to  possess  dimensions  such  as  are  suitable  for 


HIGH  FREQUENCY  AND  HIGH  POTENTIAL  CURRENTS.     315 

the  maintenance  of  a  vibration.  But  usually  the  E.  M.  r.  on  the 
circuit  or  branch  \a  will  be  something  like  a  1 00  volts  or  so,  and 
in  this  case  it  is  not  sufficient  to  break  through  the  gap.  Many 
different  means  may  be  used  to  remedy  this  by  raising  the  E.  M.  F. 
across  the  gap.  The  simplest  is  probably  to  insert  a  large  self- 
induction  coil  in  series  with  the  circuit  L.  When  the  arc  is 
established,  as  by  the  discharger  illustrated  in  B'ig.  166,  the  mag- 
net blows  the  arc  out  the  instant  it  is  formed.  Now  the  extra 
current  of  the  break,  being  of  high  E.  M.  F.,  breaks  through  the 
gap,  and  a  path  of  low  resistance  for  the  dynamo  current  being 
again  provided,  there  is  a  sudden  rush  of  current  from  the 
dynamo  upon  the  weakening  or  subsidence  of  the  extra  current. 
This  process  is  repeated  in  rapid  succession,  and  in  this  manner  I 
have  maintained  oscillation  with  as  low  as  50  volts,  or  even  less, 
across  the  gap.  But  conversion  on  this  plan  is  not  to  be  recom- 
mended on  account  of  the  too  heavy  currents  through  the  gap 
and  consequent  heating  of  the  electrodes ;  besides,  the  frequen- 
cies obtained  in  this  way  are  low,  owing  to  the  high  self-induc- 
tion necessarily  associated  with  the  circuit.  It  is  very  desirable 
to  have  the  E.  M.  F.  as  high  as  possible,  first,  in  order  to  increase 
the  economy  of  the  conversion,  and,  secondly,  to  obtain  high 
frequencies.  The  difference  of  potential  in  this  electric  oscilla- 
tion is,  of  course,  the  equivalent  of  the  stretching  force  in  the 
mechanical  vibration  of  the  spring.  To  obtain  very  rapid  vibra- 
tion in  a  circuit  of  some  inertia,  a  great  stretching  force  or  differ- 
ence of  potential  is  necessary.  Incidentally,  when  the  E.  M.  F.  is 
very  great,  the  condenser  which  is  usually  employed  in  connec- 
tion with  the  circuit  need  but  have  a  small  capacity,  and  many 
other  advantages  are  gained.  With  a  view  of  raising  the  E.  M.  F. 
to  a  many  times  greater  value  than  obtainable  from  ordinary 
distribution  circuits,  a  rotating  transformer  g  is  used,  as  indi- 
cated at  i  la,  Fig.  165,  or  else  a  separate  high  potential  machine 
is  driven  by  means  of  a  motor  operated  from  the  generator  G. 
The  latter  plan  is  in  fact  preferable,  as  changes  are  easier  made. 
The  connections  from  the  high  tension  winding  are  quite  similar 
to  those  in  branch  la  with  the  exception  that  a  condenser  c, 
which  should  be  adjustable,  is  connected  to  the  high  tension 
circuit.  Usually,  also,  an  adjustable  self-induction  coil  in  series 
with  the  circuit  has  been  employed  in  these  experiments.  When 
the  tension  of  the  currents  is  very  high,  the  magnet  ordinarily 
used  in  connection  with  the  discharger  is  of  comparatively  small 


316  INVENTIONS  OF  NIKOLA  TKSLA. 

value,  as  it  is  quite  easy  to  adjust  the  dimensions  of  the  circuit 
so  that  oscillation  is  maintained.  The  employment  of  a  steady 
E.  M.  F.  in  the  high  frequency  conversion  affords  some  advan- 
tages over  the  employment  of  alternating  E.  M.  F.,  as  the  adjust- 
ments are  much  simpler  and  the  action  can  be  easier  controlled. 
But  unfortunately  one  is  limited  by  the  obtainable  potential  dif- 
ference. The  winding  also  breaks  down  easily  in  consequence 
of  the  sparks  which  form  between  the  sections  of  the  armature 
or  commutator  when  a  vigorous  oscillation  takes  place.  Besides, 
these  transformers  are  expensive  to  build.  It  has  been  found  by 
experience  that  it  is  best  to  follow  the  plan  illustrated  at  ma. 
In  this  arrangement  a  rotating  transformer  g,  is  employed  to 
convert  the  low  tension  direct  currents  into  low  frequency  alter- 
nating currents,  preferably  also  of  small  tension.  The  tension 
of  the  currents  is  then  raised  in  a  stationary  transformer  T.  The 
secondary  s  of  this  transformer  is  connected  to  an  adjustable  con- 
denser c  which  discharges  through  the  gap  or  discharger  dd,  placed 
in  either  of  the  ways  indicated,  through  the  primary  p  of  a  dis- 
ruptive discharge  coil,  the  high  frequency  current  being  obtained 
from  the  secondary  s  of  this  coil,  as  described  on  previous  occa- 
sions. This  will  undoubtedly  be  found  the  cheapest  and  most  con- 
venient way  of  converting  direct  currents. 

The  three  branches  of  the  circuit  A  represent  the  usual  cases 
met  in  practice  when  alternating  currents  are  converted.  In 
Fig.  15  a  condenser  c.,  generally  of  large  capacity,  is  connected  to  the 
circuit  L  containing  the  devices  Z  Z,  m  m.  The  devices  mm  are  sup- 
posed to  be  of  high  self-induction  so  as  to  bring  the  frequency  of 
the  circuit  more  or  less  to  that  of  the  dynamo.  In  this  instance 
the  discharger  d  d  should  best  have  a  number  of  makes  and  breaks 
per  second  equal  to  twice  the  frequency  of  the  dynamo.  If  not 
so,  then  it  should  have  at  least  a  number  equal  to  a  multiple  or 
even  fraction  of  the  dynamo  frequency.  It  should  be  observed, 
referring  to  iJ,  that  the  conversion  to  a  high  potential  is  also 
effected  when  the  discharger  d  d,  which  is  shown  in  the  sketch,  is 
omitted.  But  the  effects  which  are  produced  by  currents  which 
rise  instantly  to  high  values,  as  in  a  disruptive  discharge,  are 
entirely  different  from  those  produced  by  dynamo  currents  which 
rise  and  fall  harmonically.  So,  for  instance,  there  might  be  in  a 
given  case  a  number  of  makes  and  breaks  at  d  d  equal  to  just 
twice  the  frequency  of  the  dynamo,"or  in  other  words,  there  may 
be  the  same  number  of  fundamental  oscillations  as  would  be  pro- 


HIGH  FREQUENCY  AND  HIGH  POTENTIAL  CURBENT8.     317 

duced  without  the  discharge  gap,  and  there  might  even  not  be  any 
quicker  superimposed  vibration  ;  yet  the  differences  of  potential  at 
the  various  points  of  the  circuit,  the  impedance  and  other  pheno- 
mena, dependent  upon  the  rate  of  change,  will  bear  no  similarity  in 
the  two  cases.  Thus,  when  working  with  currents  discharging  dis- 
ruptively,  the  element  chiefly  to  be  considered  is  not  the  frequency, 
as  a  student  might  be  apt  to  believe,  but  the  rate  of  change  per 
unit  of  time.  With  low  frequencies  in  a  certain  measure  the  same  . 
effects  may  be  obtained  as  with  high  frequencies,  provided  the  rate 
of  change  is  sufficiently  great.  So  if  a  low  frequency  current  is 
raised  to  a  potential  of,  say,  75,000  volts,  and  the  high  tension  cur- 
rent passed  through  a  series  of  high  resistance  lamp  filaments,  the 
importance  of  the  rarefied  gas  surrounding  the  filament  is  clearly 
noted,  as  will  be  seen  later;  or,  if  a  low  frequency  current  of  several 
thousand  amperes  is  passed  through  a  metal  bar,  striking  phe- 
nomena of  impedance  are  observed,  just  as  with  currents  of  high 
frequencies.  But  it  is,  of  course,  evident  that  with  low  frequency 
currents  it  is  impossible  to  obtain  such  rates  of  change  per  unit  of 
time  as  with  high  frequencies,  hence  the  effects  produced  by  the 
latter  are  much  more  prominent.  It  is  deemed  advisable  to 
make  the  preceding  remarks,  inasmuch  as  many  more  recently 
described  effects  have  been  unwittingly  identified  with  high 
frequencies.  Frequency  alone  in  reality  does  not  mean  anything, 
except  when  an  undisturbed  harmonic  oscillation  is  considered. 

In  the  branch  uib  a  similar  disposition  to  that  in  ib  is  illustrated, 
with  the  difference  that  the  currents  discharging  through  the  gap 
d  d  are  used  to  induce  currents  in  the  secondary  s  of  a  trans- 
former T.  In  such  case  the  secondary  should  be  provided  with  an 
adjustable  condenser  for  the  purpose  of  tuning  it  to  the  primary. 

lib  illustrates  a  plan  of  alternate  current  high  frequency 
conversion  which  is  most  frequently  used  and  which  is  found  to 
be  most  convenient.  This  plan  has  been  dwelt  upon  in  detail  on 
previous  occasions  and  need  not  be  described  here. 

Some  of  these  results  were  obtained  by  the  use  of  a  high 
frequency  alternator.  A  description  of  such  machines  will  be 
found  in  my  original  paper  before  the  American  Institute  of 
Electrical  Engineers,  and  in  periodicals  of  that  period,  notably 
in  THE  ELECTRICAL  ENGINEER  of  March  18,  1891. 

I  will  now  proceed  with  the  experiments. 


318  INVENTIONS  OF  NIKOLA  TESLA. 

ON    PHENOMENA    PRODUCED    BY    ELECTROSTATIC    FORCE. 

The  first  class  of  effects  I  intend  to  show  you  are  effects  pro- 
duced by  electrostatic  force.  It  is  the  force  which  governs  the 
the  motion  of  the  atoms,  which  causes  them  to  collide  and  de- 
velop the  life-sustaining  energy  of  heat  and  light,  and  which 
causes  them  to  aggregate  in  an  in  finite  variety  of  ways,  according 
to  Nature's  fanciful  designs,  and  to  form  all  these  wondrous 
structures  we  perceive  around  us ;  it  is,  in  fact,  if  our  present 
views  be  true,  the  most  important  force  for  us  to  consider  in  Na- 
ture. As  the  term  electrostatic  might  imply  a  steady  electric 
condition,  it  should  be  remarked,  that  in  these  experiments  the 
force  is  not  constant,  but  varies  at  a  rate  which  may  be  consid- 
ered moderate,  about  one  million  times  a  second,  or  thereabouts. 
This  enables  me  to  produce  many  effects  which  are  not  produ- 
cible with  an  unvarying  force. 

When  two  conducting  bodies  are  insulated  and  electrified, 
we  say  that  an  electrostatic  force  is  acting  between  them.  This 
force  manifests  itself  in  attractions,  repulsions  and  stresses  in  the 
bodies  and  space  or  medium  without.  So  great  may  be  the  strain 
exerted  in  the  air,  or  whatever  separates  the  two  conducting 
bodies,  that  it  may  break  down,  and  we  observe  sparks  or  bundles 
of  light  or  streamers,  as  they  are  called.  These  streamers  form 
abundantly  when  the  force  through  the  air  is  rapidly  varying.  I 
will  illustrate  this  action  of  electrostatic  force  in  a  novel  experi- 
ment in  which  I  will  employ  the  induction  coil  before  referred 
to.  The  coil  is  contained  in  a  trough  filled  with  oil,  and  placed 
under  the  table.  The  two  ends*  of  the  secondary  wire  pass 
through  the  two  thick  columns  of  hard  rubber  which  protrude 
to  some  height  above  the  table.  It  is  necessary  to  insulate  the 
ends  or  terminals  of  the  secondary  heavily  with  hard  rubber,  be- 
cause even  dry  wood  is  by  far  too  poor  an  insulator  for  these  cur- 
rents of  enormous  potential  differences.  On  one  of  the  termi- 
nals of  the  coil,  I  have  placed  a  large  sphere  of  sheet  brass,  which 
is  connected  to  a  larger  insulated  brass  plate,  in  order  to  enable 
me  to  perform  the  experiments  under  conditions,  which,  as  you 
will  see,  are  more  suitable  for  this  experiment.  I  now  set  the 
coil  to  work  and  approach  the  free  terminal  with  a  metallic  ob- 
ject held  in  my  hand,  this  simply  to  avoid  burns.  As  I  approach  the 
metallic  object  to  a  distance  of  eight  or  ten  inches,  a  torrent  of  furi- 
ous sparks  breaks  forth  from  the  end  of  the  secondary  wire,  which 


HIGH  FREQUENCY  AND  HIGH  POTENTIAL  CURRENTS.      319 

passes  through  the  rubber  column.  The  sparks  cease  when  the 
metal  in  my  hand  touches  the  wire.  My  arm  is  now  traversed 
by  a  powerful  electric  current,  vibrating  at  about  the  rate  of  one 
million  times  a  second.  All  around  me  the  electrostatic  force 
makes  itself  felt,  and  the  air  molecules  and  particles  of  dust  flying 
about  are  acted  upon  and  are  hammering  violently  against  my 
body.  So  great  is  this  agitation  of  the  particles,  that  when  the 
lights  are  turned  out  you  may  see  streams  of  feeble  light  appear 
on  some  parts  of  my  body.  When  such  a  streamer  breaks  out  on 
any  part  of  the  body,  it  produces  a  sensation  like  the  pricking  of 
a  needle.  Were  the  potentials  sufficiently  high  and  the  frequency 
of  the  vibration  rather  low,  the  skin  would  probably  be  rup- 
tured under  the  tremendous  strain,  and  the  blood  would  rush  out 
with  great  force  in  the  form  of  fine  spray  or  jet  so  thin  as  to  be 
invisible,  just  as  oil  will  when  placed  on  the  positive  terminal  of 


FIG.  169. 

a  Holtz  machine.  The  breaking  through  of  the  skin  though  it 
may  seem  impossible  at  first,  would  perhaps  occur,  by  reason  of 
the  tissues  under  the  skin  being  incomparably  better  conducting. 
This,  at  least,  appears  plausible,  judging  from  some  observations. 
I  can  make  these  streams  of  light  visible  to  all,  by  touching 
with  the  metallic  object  one  of  the  terminals  as  before,  and 
approaching  my  free  hand  to  the  brass  sphere,  which  is  con- 
nected to  the  second  terminal  of  the  coil.  As  the  hand  is 
approached,  the  air  between  it  and  the  sphere,  or  in  the  imme- 
diate neighborhood,  is  more  violently  agitated,  and  you  see 
streams  of  light  now  break  forth  from  my  finger  tips  and 
from  the  whole  hand  (Fig.  169).  Were  I  to  approach  the  hand 
closer,  powerful  sparks  would  jump  from  the  brass  sphere  to 
my  hand,  which  might  be  injurious.  The  streamers  offer  no 
particular  inconvenience,  except  that  in  the  ends  of  the  finger 


820  INVENTIONS  OF  NIKOLA  TE8LA. 

tips  a  burning  sensation  is  felt.  They  should  not  be  confounded 
with  those  produced  by  an  influence  machine,  because  in  many 
respects  they  behave  differently.  I  have  attached  the  brass  sphere 
and  plate  to  one  of  the  terminals  in  order  to  prevent  the  formation 
of  visible  streamers  on  that  terminal,  also  in  order  to  prevent 
sparks  from  jumping  at  a  considerable  distance.  Besides,  the 
attachment  is  favorable  for  the  working  of  the  coil. 

The  streams  of  light  which  you  have  observed  issuing  from  my 
hand  are  due  to  a  potential  of  about  200,000  volts,  alternating  in 
rather  irregular  intervals,  sometimes  like  a  million  times  a  second. 
A  vibration  of  the  same  amplitude,  but  four  times  as  fast,  to  main- 
tain which  over  3,000,000  volts  would  be  required,  would  be 
more  than  sufficient  to  envelop  my  body  in  a  complete  sheet  of 
flame.  But  this  flame  would  not  burn  me  up  ;  quite  contrarily, 
the  probability  is  that  I  would  not  be  injured  in  the  least.  Yet  a 
hundredth  part  of  that  energy,  otherwise  directed,  would  be  amply 
sufficient  to  kill  a  person. 

The  amount  of  energy  which  may  thus  be  passed  into  the  body 
of  a  person  depends  on  the  frequency  and  potential  of  the  cur- 
rents, and  by  making  both  of  these  very  great,  a  vast  amount  of 
energy  may  be  passed  into  the  body  without  causing  any  discom- 
fort, except  perhaps,  in  the  arm,  which  is  traversed  by  a  true 
conduction  current.  The  reason  why  no  pain  in  the  body  is  felt, 
and  no  injurious  effect  noted,  is  that  everywhere,  if  a  current  be 
imagined  to  flow  through  the  body,  the  direction  of  its  flow 
would  be  at  right  angles  to  the  surface ;  hence  the  body  of  the 
experimenter  offers  an  enormous  section  to  the  current,  and  the 
density  is  very  small,  with  the  exception  of  the  arm,  perhaps, 
where  the  density  may  be  considerable.  But  if  only  a  small 
fraction  of  that  energy  would  be  applied  in  such  a  way  that  a  cur- 
rent would  traverse  the  body  in  the  same  manner  as  a  low  fre- 
quency current,  a  shock  would  be  received  which  might  be  fatal. 
A  direct  or  low  frequency  alternating  current  is  fatal,  I  think, 
principally  because  its  distribution  through  the  body  is  not 
uniform,  as  it  must  divide  itself  in  minute  streamlets  of  great 
density,  whereby  some  organs  are  vitally  injured.  That  such  a 
process  occurs  I  have  not  the  least  doubt,  though  no  evidence 
might  apparently  exist,  or  be  found  upon  examination.  The 
surest  to  injure  and  destroy  life,  is  a  continuous  current,  but  the 
most  painful  is  an  alternating  current  of  very  low  frequency. 
The  expression  of  these  views,  which  are  the  result  of  long  con- 


HIGH  FREQUENCY  AND  HIGH  POTENTIAL  CURRENTS.    321 

tinued  experiment  and  observation,  both  with  steady  and  varying 
currents,  is  elicited  by  the  interest  which  is  at  present  taken  in 
this  subject,  and  by  the  manifestly  erroneous  ideas  which  are 
daily  propounded  in  journals  on  this  subject. 

I  may  illustrate  an  effect  of  the  electrostatic  force  by  another 
striking  experiment,  but  before,  I  must  call  your  attention  to  one 
or  two  facts.  I  have  said  before,  that  when  the  medium  be- 
tween two  oppositely  electrified  bodies  is  strained  beyond  a  cer- 
tain limit  it  gives  way  and,  stated  in  popular  language,  the 
opposite  electric  charges  unite  and  neutralize  each  other.  This 
breaking  down  of  the  medium  occurs  principally  when  the  force 
acting  between  the  bodies  is  steady,  or  varies  at  a  moderate  rate. 
Were  the  variation  sufficiently  rapid,  such  a  destructive  break 
would  not  occur,  no  matter  how  great  the  force,  for  all  the  en- 
ergy would  be  spent  in  radiation,  convection  and  mechanical  and 
chemical  action.  Thus  the  spark  length,  or  greatest  distance 
which  a  spark  will  jump  between  the  electrified  bodies  is  the 


FIG.  170a.         FIG.  170b. 

smaller,  the  greater  the  variation  or  time  rate  of  change.  But 
this  rule  may  be  taken  to  be  true  only  in  a  general  way,  when 
comparing  rates  which  are  widely  different. 

I  will  show  you  by  an  experiment  the  difference  in  the  effect 
produced  by  a  rapidly  varying  and  a  steady  or  moderately  vary- 
ing force.  I  have  here  two  large  circular  brass  plates  p  p  (Fig. 
170#  and  Fig.  1706),  supported  on  movable  insulating  stands  on 
the  table,  connected  to  the  ends  of  the  secondary  of  a  coil  similar 
to  the  one  used  before.  I  place  the  plates  ten  or  twelve  inches 
apart  and  set  the  coil  to  work.  You  see  the  whole  space  between 
the  plates,  nearly  two  cubic  feet,  filled  with  uniform  light,  Fig. 
170«.  This  light  is  due  to  the  streamers  you  have  seen  in  the  first 
experiment,  which  are  now  much  more  intense.  I  have  already 
pointed  out  the  importance  of  these  streamers  in  commercial  ap- 
paratus and  their  still  greater  importance  in  some  purely  scien- 
tific investigations.  Often  they  are  too  weak  to  be  visible,  but 


322  INVENTIONS  OF  NIKOLA  TESLA. 

they  always  exist,  consuming  energy  and  modifying  the  action 
of  the  apparatus.  When  intense,  as  they  are  at  present,  they 
produce  ozone  in  great  quantity,  and  also,  as  Professor  Crookes 
has  pointed  out,  nitrous  acid.  So  quick  is  the  chemical  action  that 
if  a  coil,  such  as  this  one,  is  worked  for  a  very  long  time  it  will 
make  the  atmosphere  of  a  small  room  unbearable,  for  the  eyes 
and  throat  are  attacked.  But  when  moderately  produced,  the 
streamers  refresh  the  atmosphere  wonderfully,  like  a  thunder- 
storm, and  exercises  unquestionably  a  beneficial  effect. 

In  this  experiment  the  force  acting  between  the  plates  changes 
in  intensity  and  direction  at  a  very  rapid  rate.  I  will  now  make 
the  rate  of  change  per  unit  time  much  smaller.  This  I  effect  by 
rendering  the  discharges  through  the  primary  of  the  induction 
coil  less  frequent,  and  also  by  diminishing  the  rapidity  of  the  vi- 
bration in  the  secondary.  The  former  result  is  conveniently  se- 
cured by  lowering  the  E.  M.  r.  over  the  air  gap  in  the  primary 
circuit,  the  latter  by  approaching  the  two  brass  plates  to  a  dis- 
tance of  about  three  or  four  inches.  When  the  coil  is  set  to  work, 
you  see  no  streamers  or  light  between  the  plates,  yet  the  medium 
between  them  is  under  a  tremendous  strain.  I  still  further  aug- 
ment the  strain  by  raising  the  E.  M.  F.  in  the  primary  circuit,  and 
soon  you  see  the  air  give  way  and  the  hall  is  illuminated  by  a 
shower  of  brilliant  and  noisy  sparks,  Fig.  1TO&.  These  sparks  could 
be  produced  also  with  unvarying  force  ;  they  have  been  for  many 
years  a  familiar  phenomenon,  though  they  were  usually  obtained 
from  an  entirely  different  apparatus.  In  describing  these  two 
phenomena  so  radically  different  in  appearance,  I  have  advisedly 
spoken  of  a  "  force  "  acting  between  the  plates.  It  would  be  in 
accordance  with  accepted  views  to  say,  that  there  was  an  "  alter- 
nating E.  M.  F,"  acting  between  the  plates.  This  term  is  quite 
proper  and  applicable  in  all  cases  where  there  is  evidence  of  at 
least  a  possibility  of  an  essential  inter-dependence  of  the  electric 
state  of  the  plates,  or  electric  action  in  their  neighborhood.  But 
if  the  plates  were  removed  to  an  infinite  distance,  or  if  at  a  finite 
distance,  there  is  no  probability  or  necessity  whatever  for  such 
dependence.  I  prefer  to  use  the  term  "  electrostatic  force,"  and 
to  say  that  such  a  force  is  acting  around  each  plate  or  electrified  in- 
sulated body  in  general.  There  is  an  inconvenience  in  using  this 
express/on  as  the  term  incidentally  means  a  steady  electric  con- 
dition ;  but  a  proper  nomenclature  will  eventually  settle  this  dif- 
ficulty. 


HIGH  FItEQ  UENCY  AND  HIQII  POTENTIAL  CUKRENTS.     323 

I  now  return  to  the  experiment  to  which  I  have  already  al- 
luded, and  with  which  I  desire  to  illustrate  a  striking  effect  pro- 
duced by  a  rapidly  varying  electrostatic  force.  I  attach  to  the  end 
of  the  wire,  I  (Fig.  171),  which  is  in  connection  with  one  of  the 
terminals  of  the  secondary  of  the  induction  coil,  an  exhausted 
bulb  I.  This  bulb  contains  a  thin  carbon  filament/,  which  is 
fastened  to  a  platinum  wire  w,  sealed  in  the  glass  and  leading 
outside  of  the  bulb,  where  it  connects  to  the  wire  I.  The 
bulb  may  be  exhausted  to  any  degree  attainable  with  ordinary 
apparatus.  Just  a  moment  before,  you  have  witnessed  the  break- 
ing down  of  the  air  between  the  charged  brass  plates.  You  know 
that  a  plate  of  glass,  or  any  other  insulating  material,  would  break 
down  in  like  manner.  Had  I  therefore  a  metallic  coating  at- 
tached to  the  outside  of  the  bulb,  or  placed  near  the  same,  and 


FIG.  171. 


FIG.  172a 


FIG.  172b. 


were  this  coating  connected  to  the  other  terminal  of  the  coil,  you 
would  be  prepared  to  see  the  glass  give  way  if  the  strain  were 
sufficiently  increased.  Even  were  the  coating  not  connected  to 
the  other  terminal,  but  to  an  insulated  plate,  still,  if  you  have 
followed  recent  developments,  you  would  naturally  expect  a  rup- 
ture of  the  glass. 

But  it  will  certainly  surprise  you  to  note  that  under  the  action 
of  the  varying  electrostatic  force,  the  glass  gives  way  when  all 
other  bodies  are  removed  from  the  bulb.  In  fact,  all  the  sur- 
rounding bodies  we  perceive  might  be  removed  to  an  infinite  dis- 
tance without  affecting  the  result  in  the  slightest.  Wher^he  coil 
is  set  to  work,  the  glass  is  invariably  broken  through  at  the  seal, 
or  other  narrow  channel,  and  the  vacuum  is  quickly  impaired. 


324  INVENTIONS  OF  NIKOLA  TKSLA. 

Such  a  damaging  break  would  not  occur  with  a  steady  force,  even 
if  the  same  were  many  times  greater.  The  break  is  due  to  the 
agitation  of  the  molecules  of  the  gas  within  the  bulb,  and  outside 
of  the  same.  This  agitation,  which  is  generally  most  violent  in 
the  narrow  pointed  channel  near  the  seal,  causes  a  heating  and 
rupture  of  the  glass.  This  rupture,  would,  however,  not  occur, 
not  even  with  a  varying  force,  if  the  medium  filling  the  inside  of 
the  bulb,  and  that  surrounding  it,  were  perfectly  homogeneous. 
The  break  occurs  much  quicker  if  the  top  of  the  bulb  is  drawn 
out  into  a  h'ne  fibre.  In  bulbs  used  with  these  coils  such  nar- 
row, pointed  channels  must  therefore  be  avoided. 

When  a  conducting  body  is  immersed  in  air,  or  similar  insulat- 
ing medium,  consisting  of,  or  containing,  small  freely  movable 
particles  capable  of  being  electrified,  and  when  the  electrification 
of  the  body  is  made  to  undergo  a  very  rapid  change — which  is 
equivalent  to  saying  that  the  electrostatic  force  acting  around 
the  body  is  varying  in  intensity, — the  small  particles  are  attracted 
and  repelled,  and  their  violent  impacts  against  the  body  may 
cause  a  mechanical  motion  of  the  latter.  Phenomena  of  this 
kind  are  noteworthy,  inasmuch  as  they  have  not  been  observed 
before  with  apparatus  such  as  has  been  commonly  in  use.  If  a 
very  light  conducting  sphere  be  suspended  on  an  exceedingly  fine 
wire,  and  charged  to  a  steady  potential,  however  high,  the  sphere 
will  remain  at  rest.  Even  if  the  potential  would  be  rapidly 
varying,  provided  that  the  small  particles  of  matter,  molecules  or 
atoms,  are  evenly  distributed,  no  motion  of  the  sphere  should  re- 
sult. But  if  one  side  of  the  conducting  sphere  is  covered  with  a 
thick  insulating  layer,  the  impacts  of  the  particles  will  cause  the 
sphere  to  move  about,  generally  in  irregular  curves,  Fig.  172&. 
In  like  manner,  as  I  have  shown  on  a  previous  occasion,  a  fan  of 
sheet  metal,  Fig.  1 72&,  covered  partially  with  insulating  material 
as  indicated,  and  placed  upon  the  terminal  of  the  coil  so  as  to  turn 
freely,  on  it,  is  spun  around. 

All  these  phenomena  you  have  witnessed  and  others  which 
will  be  shown  later,  are  due  to  the  presence  of  a  medium  like 
air,  and  would  not  occur  in  a  continuous  medium.  The  action 
of  the  air  may  be  illustrated  still  better  by  the  following  experi- 
ment. I  take  a  glass  tube  t,  Fig.  173,  of  about  an  inch  in  di- 
ameter, which  has  a  platinum  wire  w  sealed  in  the  lower  end, 
and  to  which  is  attached  a  thin  lamp  filament  f.  I  connect  the 
wire  with  the  terminal  of  the  coil  and  set  the  coil  to  work.  The 


HIGH  FREQUENCY  AND  HIGH  POTENTIAL  CURRENTS.     325 

platinum  wire  is  now  electrified  positively  and  negatively 
in  rapid  succession  and  the  wire  and  air  inside  of  the  tube 
is  rapidly  heated  by  the  impacts  of  the  particles,  which  may  be 
so  violent  as  to  render  the  filament  incandescent.  But  if  I  pour 
oil  in  the  tube,  just  as  soon  as  the  wire  is  covered  with  the  oil, 
all  action  apparently  ceases  and  there  is  no  marked  evidence  of 
heating.  The  reason  of  this  is  that  the  oil  is  a  practically  con- 
tinuous medium.  The  displacements  in  such  a  continuous  medium 
are,  with  these  frequencies,  to  all  appearance  incomparably 
smaller  than  in  air,  hence  the  work  performed  in  such  a  medium 
is  insignificant.  But  oil  would  behave  very  differently  with  fre- 
quencies many  times  as  great,  for  even  though  the  displacements 


FIG.  178. 


FIG.  174. 


be  small,  if  the  frequency  were  much  greater,  considerable  work 
might  be  performed  in  the  oil. 

The  electrostatic  attractions  and  repulsions  between  bodies  of 
measurable  dimensions  are,  of  all  the  manifestations  of  this  force, 
the  first  so-called  electrical  phenomena  noted.  But  though  they 
have  been  known  to  us  for  many  centuries,  the  precise  nature  of 
the  mechanism  concerned  in  these  actions  is  still  unknown  to  us, 
and  has  not  been  even  quite  satisfactorily  explained.  What  kind 
of  mechanism  must  that  be  ?  We  cannot  help  wondering  when 
we  observe  two  magnets  attracting  and  repelling  each  other  with 
a  force  of  hundreds  of  pounds  with  apparently  nothing  between 
them.  We  have  in  our  commercial  dynamos  magnets  capable  of 
sustaining  in  mid-air  tons  of  weight.  But  what  are  even  these 


326  INVENTIONS  OF  NIKOLA  TESLA. 

forces  acting  between  magnets  when  compared  with  the  tremen- 
dous attractions  and  repulsions  produced  by  electrostatic  force,  to 
which  there  is  apparently  no  limit  as  to  intensity.  In  lightning 
discharges  bodies  are  often  charged  to  so  high  a  potential  that 
they  are  thrown  away  with  inconceivable  force  and  torn  asunder 
or  shattered  into  fragments.  Still  even  such  effects  cannot  com- 
pare with  the  attractions  and  repulsions  which  exist  between 
charged  molecules  or  atoms,  and  which  are  sufficient  to  project 
them  with  speeds  of  many  kilometres  a  second,  so  that  under  their 
violent  impact  bodies  are  rendered  highly  incandescent  and  are 
volatilized.  It  is  of  special  mtev-  ,t  for  the  thinker  who  inquires 
into  the  nature  of  these  forces  I./  note  that  whereas  the  actions 
between  individual  molecules  (  •  atoms  occur  seemingly  under  any 
conditions,  the  attractions  and  repulsions  of  bodies  of  measurable 
dimensions  imply  a  medium  possessing  insulating  properties.  So, 
if  air,  either  by  being  rarefied  or  heated,  is  rendered  more  or  less 
conducting,  these  actions  between  two  electrified  bodies  practically 
cease,  while  the  actions  between  the  individual  atoms  continue  to 
manifest  themselves. 

An  experiment  may  serve  as  an  illustration  and  as  a  means  of 
bringing  out  other  features  of  interest.  Some  time  ago  I  showed 
that  a  lamp  filament  or  wire  mounted  in  a  bulb  and  connected  to 
one  of  the  terminals  of  a  high  tension  secondary  coil  is  set  spin- 
ning, the  top  of  the  filament  generally  describing  a  circle.  This 
vibration  was  very  energetic  when  the  air  in  the  bulb  was  at 
ordinary  pressure  and  became  less  energetic  when  the  air  in  the 
bulb  was  strongly  compressed.  It  ceased  altogether  when  the  air 
was  exhausted  so  as  to  become  comparatively  good  conducting.  I 
found  at  that  time  that  no  vibration  took  place  when  the  bulb 
was  very  highly  exhausted.  But  I  conjectured  that  the  vibration 
which  I  ascribed  to  the  electrostatic  action  between  the  walls  of 
the  bulb  and  the  filament  should  take  place  also  in  a  highly 
exhausted  bulb.  To  test  this  under  conditions  which  were  inore 
favorable,  a  bulb  like  the  one  in  Fig.  174,  was  constructed.  It 
comprised  a  globe  5,  in  the  neck  of  which  was  sealed  a  platinum 
wire  w  carrying  a  thin  lamp  filament/.  In  the  lower  part  of 
the  globe  a  tube  t  was  sealed  so  as  to  surround  the  filament.  The 
exhaustion  was  carried  as  far  as  it  was  practicable  with  .the  appa- 
ratus employed. 

This  bulb  verified  my  expectation,  for  the  filament  was  set 
spinning  when  the  current  was  turned  on,  and  became  incandes- 


HIGH  FREQUENCY  AND  HIGH  POTENTIAL  CURRENTS.      327 

cent.  It  also  showed  another  interesting  feature,  bearing  upon 
the  preceding  remarks,  namely,  when  the  filament  had  been 
kept  incandescent  some  time,  the  narrow  tube  and  the  space  in- 
side were  brought  to  an  elevated  temperature,  and  as  the  gas  in 
the  tube  then  became  conducting,  the  electrostatic  attraction  be- 
tween the  glass  and  the  filament  became  very  weak  or  ceased,  and 
the  filament  came  to  rest.  When  it  came  to  rest  it  would  glow 
far  more  intensely.  This  was  probably  due  to  its  assuming  the 
position  in  the  centre  of  the  tube  where  the  molecular  bombard- 
ment was  most  intense,  and  also  partly  to  the  fact  that  the  indi- 
vidual impacts  were  more  violent  and  that  no  part  of  the  supplied 
energy  was  converted  into  mechanical  movement.  Since,  in  ac- 
cordance with  accepted  views,  in  this  experiment  the  incandescence 
must  be  attributed  to  the  impacts  of  the  particles,  molecules  or 
atoms  in  the  heated  space,  these  particles  must  therefore,  in  order 
to  explain  such  action,  be  assumed  to  behave  as  independent  car- 
riers of  electric  charges  immersed  in  an  insulating  medium  ;  yet 
there  is  no  attractive  force  between  the  glass  tube  and  the  fila- 
ment because  the  space  in  the  tube  is,  as  a  whole,  conducting. 

It  is  of  some  interest  to  observe  in  this  connection  that  whereas 
the  attraction  between  two  electrified  bodies  may  cease  owing  to 
the  impairing  of  the  insulating  power  of  the  medium  in  which 
they  are  immersed,  the  repulsion  between  the  bodies  may  still  be 
observed.  This  may  be  explained  in  a  plausible  way.  When  the 
bodies  are  placed  at  some  distance  in  a  poorly  conducting  medium, 
such  as  slightly  warmed  or  rarefied  air,  and  are  suddenly  electri- 
fied, opposite  electric  charges  being  imparted  to  them,  these 
charges  equalize  more  or  less  by  leakage  through  the  air.  But  if 
the  bodies  are  similarly  electrified,  there  is  less  opportunity  af- 
forded for  such  dissipation,  hence  the  repulsion  observed  in  such 
case  is  greater  than  the  attraction.  Repulsive  actions  in  a  gas- 
eous medium  are  however,  as  Prof.  Crookes  has  shown,  enhanced 
by  molecular  bombardment. 

ON    CURRENT   OR   DYNAMIC    ELECTRICITY    PHENOMENA. 

So  far,  I  have  considered  principally  effects  produced  by  a 
varying  electrostatic  force  in  an  insulating  medium,  such  as  air. 
When  such  a  force  is  acting  upon  a  conducting  body  of  measur- 
able dimensions,  it  causes  within  the  same,  or  on  its  surface, 
displacements  of  the  electricity  and  gives  rise  to  electric  currents, 
and  these  produce  another  kind  of  phenomena,  some  of  which  I 


328  INVENTIONS  OF  NIKOLA  TKSLA. 

shall  presently  endeavor  to  illustrate.  In  presenting  this  second 
class  of  electrical  effects,  I  will  avail  myself  principally  of  such 
as  are  producible  without  any  return  circuit,  hoping  to  interest 
you  the  more  by  presenting  these  phenomena  in  a  more  or  less 
novel  aspect. 

It  has  been  a  long  time  customary,  owing  to  the  limited 
experience  with  vibratory  currents,  to  consider  an  electric  cur- 
rent as  something  circulating  in  a  closed  conducting  path.  It 
was  astonishing  at  first  to  realize  that  a  current  may  flow  through 
the  conducting  path  even  if  the  latter  be  interrupted,  and  it 
was  still  more  surprising  to  learn,  that  sometimes  it  may  be 
even  easier  to  make  a  current  flow  under  such  conditions 
than  through  a  closed  path.  But  that  old  idea  is  gradually  dis 
appearing,  even  among  practical  men,  and  will  soon  be  entirely 
forgotten. 

If  I  connect  an  insulated  metal  plate  P,  Fig.  175,  to  one  of  the 
terminals  T  of  the  induction  coil  by  means  of  a  wire,  though  this 


FIG.  175. 

plate  be  very  well  insulated,  a  current  passes  through  the 
wire  when  the  coil  is  set  to  work.  First  I  wish  to  give  you 
evidence  that  there  is  a  current  passing  through  the  connecting 
wire.  An  obvious  way  of  demonstrating  this  is  to  insert  between 
the  terminal  of  the  coil  and  the  insulated  plate  a  very  thin  plati- 
num or  german  silver  wire  w  and  bring  the  latter  to  incandes- 
cence or  fusion  by  the  current.  This  requires  a  rather  large  plate 
or  else  current  impulses  of  very  high  potential  and  frequency. 
Another  way  is  to  take  a  coil  c,  Fig.  175,  containing  many  turns  of 
thin  insulated  wire  and  to  insert  the  same  in  the  path  of  the  cur- 
rent to  the  plate.  When  I  connect  one  of  the  ends  of  the  coil  to  the 
wire  leading  to  another  insulated  plate  pl5  and  its  other  end  to  the 
terminal  TJ  of  the  induction  coil,  and  set  the  latter  to  work,  a  cur- 
rent passes  through  the  inserted  coil  c  and  the  existence  of  the 
current  may  be  made  manifest  in  various  ways.  For  instance,  I 


HIGH  FREQUENCY  AND  HIGH  POTENTIAL  CURRENTS.      329 

insert  an  iron  core  *  within  the  coil.  The  current  being  one  of 
very  high  frequency,  will,  if  it  be  of  some  strength,  soon  bring  the 
iron  core  to  a  noticeably  higher  temperature,  as  the  hysteresis  and 
current  losses  are  great  witli  such  high  frequencies.  One  might 
take  a  core  of  some  size,  laminated  or  not,  it  would  matter  little ; 
but  ordinary  iron  wire  -^th  or  £th  of  an  inch  thick  is  suitable 
for  the  purpose.  While  the  induction  coil  is  working,  a  current 
traverses  the  inserted  coil  and  only  a  few  moments  are  sufficient 
to  bring  the  iron  wire  i  to  an  elevated  temperature  sufficient  to 
soften  the  sealing-wax  s,  and  cause  a  paper  washer  p  fastened  by 
it  to  the  iron  wire  to  fall  off.  But  with  the  apparatus  such  as  I 
have  here,  other,  much  more  interesting,  demonstrations  of  this 
kind  can  be  made.  I  have  a  secondary  s,  Fig  176,  of  coarse  wire, 
wound  upon  a  coil  similar  to  the  first.  In  the  preceding  experi- 
ment the  current  through  the  coil  c,  Fig.  175,  was  very  small,  but 
there  being  many  turns  a  strong  heating  effect  was,  nevertheless, 


FIG.  176. 


produced  in  the  iron  wire.  Had  I  passed  that  current  through  a 
conductor  in  order  to  show  the  heating  of  the  latter,  the  current 
might  have  been  too  small  to  produce  the  effect  desired.  But  with 
this  coil  provided  with  a  secondary  winding,  I  can  now  transform 
the  feeble  current  of  high  tension  which  passes  through  the  prim- 
ary P  into  a  strong  secondary  current  of  low  tension,  and  this 
current  will  quite  certainly  do  what  I  expect.  In  a  small  glass 
tube  (t,  Fig.  176),  I  have  enclosed  a  coiled  platinum  wire,  w,  this 
merely  in  order  to  protect  the  wire.  On  each  end  of  the  glass 
tube  is  sealed  a  terminal  of  stout  wire  to  which  one  of  the  ends  of 
the  platinum  wire  w,  is  connected.  I  join  the  terminals  of  the 
secondary  coil  to  these  terminals  and  insert  the  primary  p, 
between  the  insulated  plate  rl5  and  the  terminal  TJ,  of  the  induc- 
tion coil  as  before.  The  latter  being  set  to  work,  instantly  the 
platinum  wire  w  is  rendered  incandescent  and  can  be  fused,  even 
if  it  be  verv  thick. 


330  INVENTIONS  OF  NIKOLA  TESLA. 

Instead  of  the  platinum  wire  I  now  take  an  ordinary  50-volt 
Ifi  c.  p.  lamp.  When  I  set  the  induction  coil  in  operation  the 
lamp  filament  is  brought  to  high  incandescence.  It  is,  however, 
not  necessary  to  use  the  insulated  plate,  for  the  lamp  (7,  Fig.  177) 
is  rendered  incandescent  even  if  the  plate  pt  be  disconnected. 
The  secondary  may  also  be  connected  to  the  primary  as  indicated 
by  the  dotted  line  in  Fig.  177,  to  do  away  more  or  less  with  the 
electrostatic  induction  or  to  modify  the  action  otherwise. 

I  may  here  call  attention  to  a  number  of  interesting  observa- 
tions with  the  lamp.  First,  I  disconnect  one  of  the  terminals  of 
the  lamp  from  the  secondary  s.  When  the  induction  coil  plays, 
a  glow  is  noted  which  tills  the  whole  bulb.  This  glow  is  due  to 
electrostatic  induction.  It  increases'when  the  bulb  is  grasped 
with  the  hand,  and  the  capacity  of  the  experimenter's  body  thus 
added  to  the  secondary  circuit.  The  secondary,  in  effect,  is  equi- 
valent to  a  metallic  coating,  which  would  be  placed  near  the  pri- 


FIG.  177. 

mary .  If  the  secondary,  or  its  equivalent,  the  coating,  were  placed 
symmetrically  to  the  primary,  the  electrostatic  induction  would 
be  nil  under  ordinary  conditions,  that  is,  when  a  primary  return 
circuit  is  used,  as  both  halves  would  neutralize  each  other.  The 
secondary  is  in  fact  placed  symmetrically  to  the  primary,  but  the 
action  of  both  halves  of  the  latter,  when  only  one  of  its  ends  is 
connected  to  the  induction  coil,  is  not  exactly  equal ;  hence  elec- 
trostatic induction  takes  place,  and  hence  the  glow  in  the  bulb.  I 
can  nearly  equalize  the  action  of  both  halves  of  the  primary  by 
connecting  the  other,  free  end  of  the  same  to  the  insulated  plate, 
as  in  the  preceding  experiment.  When  the  plate  is  connected, 
the  glow  disappears.  With  a  smaller  plate  it  would  not  entirely 
disappear  and  then  it  would  contribute  to  the  brightness  of  the 
filament  when  the  secondary  is  closed,  by  warming  the  air  in  the 
bulb. 


HIGH  FREQUENCY  AND  HIGH  POTENTIAL  CURRENTS.      331 

To  demonstrate  another  interesting  feature,  I  have  adjusted 
the  coils  used  in  a  certain  way.  I  first  connect  both  the  terminals 
of  the  lamp  to  the  secondary,  one  end  of  the  primary  being  con- 
nected to  the  terminal  TJ  of  the  induction  coil  and  the  other  to 
the  insulated  plate  pt  as  before.  When  the  current  is  turned  on, 
the  lamp  glows  brightly,  as  shown  in  Fig.  17S&,  in  which  c  is  a  fine 
wire  coil  and  s  a  coarse  wire  secondary  wound  upon  it.  If  the 
insulated  plate  pt  is  disconnected,  leaving  one  of  the  ends  a  of  the 


FIG.  178b. 

primary  insulated,  the  filament  becomes  dark  or  generally  it  dim- 
inishes in  brightness  (Fig.  1780).  Connecting  again  the  plate  pt 
and  raising  the  frequency  of  the  current,  I  make  the  filament 
quite  dark  or  barely  red  (Fig.  179J).  Once  more  I  will  discon- 
nect the  plate.  One  will  of  course  infer  that  when  the  plate  is 
disconnected,  the  current  through  the  primary  will  be  weakened, 
that  therefore  the  E.  M.  F.  will  fall  in  the  secondary  s,  and  that 
the  brightness  of  the  lamp  will  diminish.  This  might  be  the 
case  and  the  result  can  be  secured  by  an  easy  adjustment  of  the 


332 


INVENTIONS  OF  NIKOLA  TESLA. 


coils  ;  also  by  varying  the  frequency  and  potential  of  the  cur- 
rents. But  it  is  perhaps  of  greater  interest  to  note,  that  the  lamp 
increases  in  brightness  when  the  plate  is  disconnected  (Fig.  179#). 
In  this  case  all  the  energy  the  primary  receives  is  now  sunk  into 
it,  like  the  charge  of  a  battery  in  an  ocean  cable,  but  most  of  that 
energy  is  recovered  through  the  secondary  and  used  to  light  the 
lamp.  The  current  traversing  the  primary  is  strongest  at  the  end 
b  which  is  connected  to  the  terminal  TX  of  the  induction  coil,  and 


FIG  179b. 

diminishes  in  strength  towards  the  remote  end  a.  But  the  dyna- 
mic inductive  effect  exerted  upon  the  secondary  s  is  now  greater 
than  before,  when  the  suspended  plate  was  connected  to  the 
primary.  These  results  might  have  been  produced  by  a  number 
of  causes.  For  instance,  the  plate  P!  being  connected,  the  reac- 
tion from  the  coil  c  may  be  such  as  to  diminish  the  potential  at 
the  terminal  Tt  of  the  induction  coil,  and  therefore  weaken  the 
current  through  the  primary  of  the  coil  c.  Or  the  disconnecting 


HIGH  FREQUENCY  AND  HIGH  POTENTIAL  CURRENTS.      333 

of  the  plate  may  diminish  the  capacity  effect  with  relation  to  the 
primary  of  the  latter  coil  to  such  an  extent  that  the  current 
through  it  is  diminished,  though  the  potential  at  the  terminal  TJ 
of  the  induction  coil  may  be  the  same  or  even  higher.  Or  the 
result  might  have  been  produced  by  the  change  of  phase  of  the 
primary  and  secondary  currents  and  consequent  reaction.  But 
the  chief  determining  factor  is  the  relation  of  the  self-induction 
and  capacity  of  coil  c  and  plate  pt  and  the  frequency  of  the  cur- 
rents. The  greater  brightness  of  the  filament  in  Fig.  179&,  is, 
however,  in  part  due  to  the  heating  of  the  rarefied  gas  in  the 
lamp  by  electrostatic  induction,  which,  as  before  remarked,  is 
greater  when  the  suspended  plate  is  disconnected. 

Still  another  feature  of  some  interest  I  may  here  bring  to  your 
attention.  When  the  insulated  plate  is  disconnected  and  the  sec- 
ondary of  the  coil  opened,  by  approaching  a  small  object  to  the 
secondary,  but  very  small  sparks  can  be  drawn  from  it,  showing 
that  the  electrostatic  induction  is  small  in  this  case.  But  upon 
the  secondary  being  closed  upon  itself  or  through  the  lamp,  the 
filament  glowing  brightly,  strong  sparks  are  obtained  from  the 
secondary.  The  electrostatic  induction  is  now  much  greater, 
because  the  closed  secondary  determines  a  greater  flow  of  current 
through  the  primary  and  principally  through  that  half  of  it  which 
is  connected  to  the  induction  coil.  If  now  the  bulb  be  grasped 
with  the  hand,  the  capacity  of  the  secondary  with  reference  to  the 
primary  is  augmented  by  the  experimenter's  body  and  the  lumi- 
nosity of  the  filament  is  increased,  the  incandescence  now  being 
due  partly  to  the  flow  of  current  through  the  filament  and 
partly  to  the  molecular  bombardment  of  the  rarefied  gas  in  the 
bulb. 

The  preceding  experiments  will  have  prepared  one  for  the  next 
following  results  of  interest,  obtained  in  the  course  of  these  in- 
vestigations. Since  I  can  pass  a  current  through  an  insulated 
wire  merely  by  connecting  one  of  its  ends  to  the  source  of  elec- 
trical energy,  since  I  can  induce  by  it  another  current,  magnetize 
an  iron  core,  and,  in  short,  perform  all  operations  as  though  a  re- 
turn circuit  were  used,  clearly  I  can  also  drive  a  motor  by  the  aid 
of  only  one  wire.  On  a  former  occasion  1  have  described  a  sim- 
ple form  of  motor  comprising  a  single  exciting  coil,  an  iron  core 
and  disc.  Fig.  180  illustrates  a  modified  way  of  operating  such 
an  alternate  current  motor  by  currents  induced  in  a  transformer 
connected  to  one  lead,  and  several  other  arrangements  of  circuits 


334  INVENTIONS  OF  NIKOLA  TE8LA. 

for  operating  a  certain  class  of  alternating  motors  founded  on  the 
action  of  currents  of  differing  phase.  In  view  of  the  present 
state  of  the  art  it  is  thought  sufficient  to  describe  these  arrange- 
ments in  a  few  words  only.  The  diagram,  Fig.  180  II.,  shows 
a  primary  coil  P,  connected  with  one  of  its  ends  to  the  line  L  lead- 
ing from  a  high  tension  transformer  terminal  TJ.  In  inductive 
relation  to  this  primary  P  is  a  secondary  s  of  coarse  wire  in  the 
circuit  of  which  is  a  coil  c.  The  currents  induced  in  the  second- 
ary energize  the  iron  core  ?',  which  is  preferably,  but  not  neces- 
sarily, subdivided,  and  set  the  metal  disc  d  in  rotation.  Such  a 
motor  M2  as  diagramatically  shown  in  Fig.  180  II.,  has  been 
called  a  "  magnetic  lag  motor,"  but  this  expression  may  be  ob- 
jected to  by  those  who  attribute  the  rotation  of  the  disc  to  eddy 
currents  circulating  in  minute  paths  when  the  core  i  is  finally 
subdivided.  In  order  to  operate  such  a  motor  effectively  on  the 
plan  indicated,  the  frequencies  should  not  be  too  high,  not  more 
than  four  or  five  thousand,  though  the  rotation  is  produced  even 
with  ten  thousand  per  second,  or  more. 

In  Fig.  180  I.,  a  motor  Mt  having  two  energizing  circuits,  A  and 
B,  is  diagrammatical ly  indicated.  The  circuit  A  is  connected  to 
the  line  L  and  in  series  with  it  is  a  primary  p,  which  may  have  its 
free  end  connected  to  an  insulated  plate  pl5  such  connection 
being  indicated  by  the  dotted  lines.  The  other  motor  circuit  B 
is  connected  to  the  secondary  s  which  is  in  inductive  relation  to 
the  primary  p.  When  the  transformer  terminal  Tt  is  alternately 
electrified,  currents  traverse  the  open  line  L  and  also  circuit  A  and 
primary  p.  The  currents  through  the  latter  induce  secondary 
currents  in  the  circuit  s,  which  pass  through  the  energizing  coil 
B  of  the  motor.  The  currents  through  the  secondary  s  and  those 
through  the  primary  p  differ  in  phase  90  degrees,  or  nearly  so,  and 
are  capable  of  rotating  an  armature  placed  in  inductive  relation 
to  the  circuits  A  and  B. 

In  Fig.  180  III.,  a  similar  motor  M3  with  two  energizing  cir- 
cuits A!  and  B!  is  illustrated.  A  primary  p,  connected  with  one 
of  its  ends  to  the  line  L  has  a  secondary  s,  which  is  preferably 
wound  for  a  tolerably  high  E.  M.  r.,  and  to  which  the  two  ener- 
gizing circuits  of  the  motor  are  connected,  one  directly  to  the 
ends  of  the  secondary  and  the  other  through  a  condenser  c,  by  the 
action  of  which  the  currents  traversing  the  circuit  At  and  Bt  are 
made  to  differ  in  phase. 

In  Fig.  180  IV.,  still  another  arrangement  is  shown.  In  this 
case  two  primaries  pt  and  P2  are  connected  to  the  line  L,  one 


HIGH  FREQUENCY  AND  HIGH  POTENTIAL  CURRENTS.      335 


-e 


336  INVENTIONS  OF  NIKOLA  TESLA. 

through  a  condenser  c  of  small  capacity,  and  the  other  directly. 
The  primaries  are  provided  witli  secondaries  st  and  s2  which  are 
in  series  with  the  energizing  circuits,  A2  and  B2  and  a  motor  M3, 
the  condenser  c  again  serving  to  produce  the  requisite  difference 
in  the  phase  of  the  currents  traversing  the  motor  circuits.  As 
such  phase  motors  with  two  or  more  circuits  are  now  well  known 
in  the  art,  they  have  been  here  illustrated  diagrammatically.  No 
difficulty  whatever  is  found  in  operating  a  motor  in  the  manner 
indicated,  or  in  similar  ways ;  and  although  such  experiments  up 
to  this  day  present  only  scientific  interest,  they  may  at  a  period 
not  far  distant,  be  carried  out  with  practical  objects  in  view. 

It  is  thought  useful  to  devote  here  a  few  remarks  to  the  sub- 
ject of  operating  devices  of  all  kinds  by  means  of  only  one  leading 
wire.  It  is  quite  obvious,  that  when  high-frequency  currents  are 
made  use  of,  ground  connections  are — at  least  when  the  E.  M.  F. 
of  the  currents  is  great — better  than  a  return  wire.  Such  ground 
connections  are  objectionable  witli  steady  or  low  frequency  cur- 
rents on  account  of  destructive  chemical  actions  of  the  former 
and  disturbing  influences  exerted  by  both  on  the  neighboring  cir- 
cuits; but  with  high  frequencies  these  actions  practically  do  not 
exist.  Still,  even  ground  connections  become  superfluous  when 
the  E.  M.  F.  is  very  high,  for  soon  a  condition  is  reached,  when  the 
current  may  be  passed  more  economically  through  open,  than 
through  closed,  conductors.  Remote  as  might  seem  an  industrial 
application  of  such  single  wire  transmission  of  energy  to  one  not 
experienced  in  such  lines  of  experiment,  it  will  not  seem  so  to 
anyone  who  for  some  time  has  carried  on  investigations  of  such 
nature.  Indeed  I  cannot  see  why  such  a  plan  should  not  be 
practicable.  Nor  should  it  be  thought  that  for  carrying  out  such 
a  plan  currents  of  very  high  frequency  are  expressly  required, 
for  just  as  soon  as  potentials  of  say  30,000  volts  are  used,  the 
single  wire  transmission  may  be  effected  with  low  frequencies, 
and  experiments  have  been  made  by  me  from  which  these  infer- 
ences are  made. 

When  the  frequencies  are  very  high  it  has  been  found  in  lab- 
oratory practice  quite  easy  to  regulate  the  effects  in  the  manner 
shown  in  diagram  Fig.  181.  Here  two  primaries  p  and  pl  are  shown, 
each  connected  with  one  of  its  ends  to  the  line  L  and  with  the 
other  end  to  the  condenser  plates  c  and  c,  respectively.  Near 
these  are  placed  other  condenser  plates  cx  and  c,,  the  former  be- 
ing connected  to  the  line  L  and  the  latter  to  an  insulated  larger 


HIGH  FREQUENCY  AND  HIGH  POTENTIAL  CURRENTS.      337 

plate  P2.  On  the  primaries  are  wound  secondaries  s  and  st,  of 
coarse  wire,  connected  to  the  devices  d  and  I  respectively.  By- 
varying  the  distances  of  the  condenser  plates  c  and  cl5  and  c  and 
ct  the  currents  through  the  secondaries  s  and  st  are  varied  in 
intensity.  The  curious  feature  is  the  great  sensitiveness,  the 
slightest  change  in  the  distance  of  the  plates  producing  consid- 
erable variations  in  the  intensity  or  strength  of  the  currents.  The 
sensitiveness  may  be  rendered  extreme  by  making  the  frequency 
such,  that  the  primary  itself,  without  any  plate  attached  to  its 
free  end,  satisfies,  in  conjunction  with  the  closed  secondary,  the 
condition  of  resonance.  In  such  condition  an  extremely  small 
change  in  the  capacity  of  the  free  terminal  produces  great  varia- 
tions. For  instance,  I  have  been  able  to  adjust  the  conditions  so 
that  the  mere  approach  of  a  person  to  the  coil  produces  a  con- 
siderable change  in  the  brightness  of  the  lamps  attached  to  the 
secondary.  Such  observations  and  experiments  possess,  of  course, 
at  present,  chiefly  scientific  interest,  but  they  may  soon  become 
of  practical  importance. 

Yery  high  frequencies  are  of  course  not  practicable  with 
motors  on  account  of  the  necessity  of  employing  iron  cores.  But 
one  may  use  sudden  discharges  of  low  frequency  and  thus  obtain 
certain  advantages  of  high-frequency  currents  without  rendering 
the  iron  core  entirely  incapable  of  following  the  changes  and 
without  entailing  a  very  great  expenditure  of  energy  in  the  core. 
I  have  found  it  quite  practicable  to  operate  with  s.uch  low  fre- 
quency disruptive  discharges  of  condensers,  alternating-current 
motors.  A  certain  class  of  such  motors  which  I  advanced  a  few 
years  ago,  which  contain  closed  secondary  circuits,  will  rotate 
quite  vigorously  when  the  discharges  are  directed  through  the 
exciting  coils.  One  reason  that  such  a  motor  operates  so  well 
with  these  discharges  is  that  the  difference  of  phase  between  the 
primary  and  secondary  currents  is  90  degrees,  which  is  generally 
not  the  case  with  harmonically  rising  and  falling  currents  of  low 
frequency.  It  might  not  be  without  interest  to  show  an  experi- 
ment with  a  simple  motor  of  this  kind,  inasmuch  as  it  is  com- 
monly thought  that  disruptive  discharges  are  unsuitable  for  such 
purposes.  The  motor  is  illustrated  in  Fig.  182.  It  comprises  a 
rather  large  iron  core  *  with  slots  on  the  top  into  which  are  em- 
bedded thick  copper  washers  c  c.  In  proximity  to  the  core  is 
a  freely -movable  metal  disc  D.  The  core  is  provided  with  a  pri- 
mary exciting  coil  ca  the  ends  a  and  b  of  which  are  connected  to 


338  INVENTIONS  OF  NIKOLA  TKSLA. 

the  terminals  of  the  secondary  s  of  an  ordinary  transformer,  the 
primary  p  of  the  latter  being  connected  to  an  alternating  distri- 
bution circuit  or  generator  o  of  low  or  moderate  frequency. 
The  terminals  of  the  secondary  s  are  attached  to  a  condenser  c 
which  discharges  through  an  air  gap  d  d  which  may  be  placed 
in  series  or  shunt  to  the  coil  cx.  When  the  conditions  are 
properly  chosen  the  disc  D  rotates  with  considerable  effort  and  the 
iron  core  i  does  not  get  very  perceptibly  hot.  With  currents  from 
a  high-frequency  alternator,  on  the  contrary,  the  core  gets  rapidly 
hot  and  the  disc  rotates  with  a  much  smaller  effort.  To  perform 
the  experiment  properly  it  should  be  first  ascertained  that  the 
disc  D  is  not  set  in  rotation  when  the  discharge  is  not  occurring 
at  d  d.  It  is  preferable  to  use  a  large  iron  core  and  a  condenser 
of  large  capacity  so  as  to  bring  the  superimposed  quicker  oscil- 
lation to  a  very  low  pitch  or  to  do  away  \vith  it  entirely.  By 
observing  certain  elementary  rules  I  have  also  found  it  practi- 
cable to  operate  ordinary  series  or  shunt  direct-current  motors 
with  such  disruptive  discharges,  and  this  can  be  done  with  or 
without  a  return  wire. 

IMPEDANCE   PHENOMENA. 

Among  the  various  current  phenomena  observed,  perhaps  the 
most  interesting  are  those  of  impedance  presented  by  conductors 
to  currents  varying  at  a  rapid  rate.  In  my  first  paper  before  the 
American  Institute  of  Electrical  Engineers,  I  have  described  a 
few  striking  observations  of  this  kind.  Thus  I  showed  that  when 
such  currents  or  sudden  dischaiges  are  passed  through  a  thick 
metal  bar  there  may  be  points  on  the  bar  only  a  few  inches  apart, 
which  have  a  sufficient  potential  difference  between  them  to 
maintain  at  bright  incandescence  an  ordinary  filament  lamp.  I 
have  also  described  the  curious  behavior  of  rarefied  gas  surround- 
ing a  conductor,  due  to  such  sudden  rushes  of  current.  These 
phenomena  have  since  been  more  carefully  studied  and  one  or 
two  novel  experiments  of  this  kind  are  deemed  of  sufficient  in- 
terest to  be  described  here. 

Referring  to  Fig.  1830,  B  and  BJ  are  very  stout  copper  bars 
connected  at  their  lower  ends  to  plates  c  and  c1?  respectively,  of  a 
condenser,  the  opposite  plates  of  the  latter  being  connected  to  the 
terminals  of  the  secondary  s  of  a  high-tension  transformer,  the 
primary  p  of  which  is  supplied  with  alternating  currents  from  an 
ordinary  low-frequency  dynamo  &  or  distribution  circuit.  The 


HIGH  FREQUENCY  AND  HIGH  POTENTIAL  CURRENTS.     339 

condenser  discharges  through  an  adjustable  gap  dd&&  usual.  By 
establishing  a  rapid  vibration  it  was  found  quite  easy  to  perform 
the  following  curious  experiment.  The  bars  B  and  Bt  were  joined 
at  the  top  by  a  low-voltage  lamp  Z3;  a  little  lower  was  placed  by 
means  of  clamps  c  c,  a  50-volt  lamp  4 ;  and  still  lower  another  100- 
volt  lamp  /! ;  and  finally,  at  a  certain  distance  below  the  latter 
lamp,  an  exhausted  tube  T.  By  carefully  determining  the  po- 
sitions of  these  devices  it  was  found  practicable  to  maintain  them 


FIQB.  183a,  183b  and  183c. 

all  at  their  proper  illuminating  power.  Yet  they  were  all  con- 
nected in  multiple  arc  to  the  two  stout  copper  bars  and  required 
widely  different  pressures.  This  experiment  requires  of  course 
some  time  for  adjustment  but  is  quite  easily  performed. 

In  Figs.  1835  and  1836',  two  other  experiments  are  illustrated 
which,  unlike  the  previous  experiment,  do  not  require  very  care- 
ful adjustments.  In  Fig.  1835,  two  lamps,  ^  and  4,  the  former  a 


340  INVENTIONS  OF  NIKOLA  TESLA. 

100-volt  and  the  latter  a  50-volt  are  placed  in  certain  positions  as 
indicated,  the  100-volt  lamp  being  below  the  50-volt  lamp.  When 
the  arc  is  playing  at  d '  d  and  the  sudden  discharges  are  passed 
through  the  bars  B  B,,  the  50-volt  lamp  will,  as  a  rule,  burn  brightly, 
or  at  least  this  result  is  easily  secured,  while  the  100-volt  lamp 
will  burn  very  low  or  remain  quite  dark.  Fig.  1835.  Now  the 
bars  B  B!  may  be  joined  at  the  top  by  a  thick  cross  bar  -^  and  it 
is  quite  easy  to  maintain  the  100-volt  lamp  at  full  candle-power 
while  the  50-volt  lamp  remains  dark,  Fig.  183c.  These  results, 
as  I  have  pointed  out  previously,  should  not  be  considered  to  be 
due  exactly  to  frequency  but  rather  to  the  time  rate  of  change 
which  may  be  great,  even  with  low  frequencies.  A  great  many 
other  results  of  the  same  kind,  equally  interesting,  especially  to 
those  who  are  only  used  to  manipulate  steady  currents,  may  be 
obtained  and  they  afford  precious  clues  in  investigating  the  na- 
ture of  electric  currents. 

In  the  preceding  experiments  I  have  already  had  occasion  to 
show  some  light  phenomena  and  it  would  now  be  proper  to  study 
these  in  particular ;  but  to  make  this  investigation  more  com- 
plete I  think  it  necessary  to  make  first  a  few  remarks  on  the 
subject  of  electrical  resonance  which  has  to  be  always  observed 
in  carrying  out  these  experiments. 

ON    ELECTRICAL    RESONANCE. 

The  effects  of  resonance  are  being  more  and  more  noted  by  engi- 
neers and  are  becoming  of  great  importance  in  the  practical  opera- 
tion of  apparatus  of  all  kinds  with  alternating  currents.  A  few 
general  remarks  may  therefore  be  made  concerning  these  effects. 
It  is  clear,  that  if  we  succeed  in  employing  the  effects  of  resonance 
practically  in  the  operation  of  electric  devices  the  return  wire  will, 
as  a  matter  of  course,  become  unnecessary,  for  the  electric  vibra- 
tion may  be  conveyed  with  one  wire  just  as  well  as,  and  sometimes 
even  better  than,  with  two.  The  question  first  to  answer  is,  then, 
whether  pure  resonance  effects  are  producible.  Theory  and  ex- 
periment both  show  that  such  is  impossible  in  Nature,  for  as  the 
oscillation  becomes  more  and  more  vigorous,  the  losses  in  the  vi- 
brating bodies  and  environing  media  rapidly  increase  and  necessa- 
rily check  the  vibration  which  otherwise  would  go  on  increasing 
forever.  It  is  a  fortunate  circumstance  that  pure  resonance  is 
not  producible,  for  if  it  were  there  is  no  telling  what  dangers 
might  not  lie  in  wait  for  the  innocent  experimenter.  But  to  a 


HIGH  FREQ  UENCT  AND  HIGH  POTENTIAL  CURRENTS.      341 

certain  degree  resonance  is  producible,  the  magnitude  of  the 
effects  being  limited  by  the  imperfect  conductivity  and  imperfect 
elasticity  of  the  media  or,  generally  stated,  by  f  rictional  losses.  The 
smaller  these  tosses,  the  more  striking  are  the  effects.  The  same 
is  the  case  in  mechanical  vibration.  A  stout  steel  bar  may  be  set 
in  vibration  by  drops  of  water  falling  upon  it  at  proper  intervals; 
and  with  glass,  which  is  more  perfectly  elastic,  the  resonance 
effect  is  still  more  remarkable,  for  a  goblet  may  be  burst  by 
singing  into  it  a  note  of  the  proper  pitch.  The  electrical  resonance 
is  the  more  perfectly  attained,  the  smaller  the  resistance-  or  the 
impedance  of  the  conducting  path  and  the  more  perfect  the  dielec- 
tric. In  a  Leyden  jar  discharging  through  a  short  stranded  cable 
of  thin  wires  these  requirements  are  probably  best  fulfilled,  and 
the  resonance  effects  are  therefore  very  prominent.  Such  is  not 
the  case  with  dynamo  machines,  transformers  and  their  circuits, 
or  with  commercial  apparatus  in  general  in  which  the  presence 
of  iron  cores  complicates  the  action  or  renders  it  impossible. 
In  regard  to  Leyden  jars .  with  which  resonance  effects  are 
frequently  demonstrated,  I  would  say  that  the  effects  observed 
are  often  attributed  but  are  seldom  due  to  true  resonance,  for 
an  error  is  quite  easily  made  in  this  respect.  This  may  be 
undoubtedly  demonstrated  by  the  following  experiment.  Take, 
for  instance,  two  large  insulated  metallic  plates  or  spheres  which 
I  shall  designate  A  and  B;  place  them  at  a  certain  small  dis- 
tance apart  and  charge  them  from  a  frictional  or  influence 
machine  to  a  potential  so  high  that  just  a  slight  increase  of  the 
difference  of  potential  between  them  will  cause  the  small  air  or 
insulating  space  to  break  down.  This  is  easily  reached  by  mak- 
ing a  few  preliminary  trials.  If  now  another  plate — fastened  on 
an  insulating  handle  and  connected  by  a  wire  to  one  of  the  ter- 
minals of  a  high  tension  secondary  of  an  induction  coil,  which 
is  maintained  in  action  by  an  alternator  (preferably  high  fre- 
quency)— is  approached  to  one  of  the  charged  bodies  A  or  B,  so  as 
to  be  nearer  to  either  one  of  them,  the  discharge  will  invariably 
occur  between  them  ;  at  least  it  will,  if  the  potential  of  the  coil 
in  connection  with  the  plate  is  sufficiently  high.  But  the  expla- 
nation of  this  will  soon  be  found  in  the  fact  that  the  approached 
plate  acts  inductively  upon  the  bodies  A  and  B  and  causes  a  spark 
to  pass  between  them.  When  this  spark  occurs,  the  charges  which 
were  previously  imparted  to  these  bodies  from  the  influence  ma- 
chine, must  needs  be  lost,  since  the  bodies  are  brought  in  electri- 


342  INVENTIONS  OF  NIKOLA  TESLA. 

cal  connection  through  the  arc  formed.  Xow  this  arc  is  formed 
whether  there  be  resonance  or  not.  But  even  if  the  spark  would 
not  be  produced,  still  there  is  an  alternating  E.  M.  F.  set  up  between 
the  bodies  when  the  plate  is  brought  near  one  of  thfem  ;  therefore 
the  approach  of  the  plate,  if  it  does  not  always  actually,  will,  at  any 
rate,  tend  to  break  down  the  air  space  by  inductive  action.  Instead 
of  the  spheres  or  plates  A  and  B  we  may  take  the  coatings  of  a  Ley- 
den  jar  with  the  same  result,  and  in  place  of  the  machine, — which 
is  a  high  frequency  alternator  preferably,  because  it  is  more  suit- 
able for  the  experiment  and  also  for  the  argument, — we  may  take 
another  Leyden  jar  or  battery  of  jars.  When  such  jars  are  dis- 
charging through  a  circuit  of  low  resistance  the  same  is  traversed 
by  currents  of  very  high  frequency.  The  plate  may  now  be  con- 
nected to  one  of  the  coatings  of  the  second  jar,  and  when  it  is 
brought  near  to  the  first  jar  just  previously  charged  to  a  high 
potential  from  an  influence  machine,  the  result  is  the  same  as  be- 
fore, and  the  first  jar  will  discharge  through  a  small  air  space 
upon  the  second  being  caused  to  discharge.  But  both  jars  and 
their  circuits  need  not  be  tuned  any  closer  than  a  basso  profundo 
is  to  the  note  produced  by  a  mosquito,  as  small  sparks  will  be  pro- 
duced through  the  air  space,  or  at  least  the  latter  will  be  consider- 
ably more  strained  owing  to  the  setting  up  of  an  alternating 
K.  M.  F.  by  induction,  which  takes  place  when  one  of  the  jars  be- 
gins to  discharge.  Again  another  error  of  a  similar  nature  is  quite 
easily  made.  If  the  circuits  of  the  two  jars  are  run  parallel  and 
close  together,  and  the  experiment  has  been  performed  of  dis- 
charging one  by  the  other,  and  now  a  coil  of  wire  be  added  to  one 
of  the  circuits  whereupon  the  experiment  does  not  succeed,  the 
conclusion  that  this  is  due  to  the  fact  that  the  circuits  are  now 
not  tuned,  would  be  far  from  being  safe.  For  the  two  circuits 
act  as  condenser  coatings  and  the  addition  of  the  coil  to  one  of 
them  is  equivalent  to  bridging  them,  at  the  point  where  the  coil 
is  placed,  by  a  small  condenser,  and  the  effect  of  the  latter  might 
be  to  prevent  the  spark  from  jumping  through  the  discharge  space 
by  diminishing  the  alternating  E.  M.  F.  acting  across  the  same. 
All  these  remarks,  and  many  more  which  might  be  added  but  for 
fear  of  wandering  too  far  from  the  subject,  are  made  with  the 
pardonable  intention  of  cautioning  the  unsuspecting  student,  who 
might  gain  an  entirely  unwarranted  opinion  of  his  skill  at  see- 
ing every  experiment  succeed ;  but  they  are  in  no  way  thrust  upon 
the  experienced  as  novel  observations. 


HIGH  FREQUENCY  AND  HIGH  POTENTIAL  CURRENTS.     843 

In  order  to  make  reliable  observations  of  electric  resonance 
effects  it  is  very  desirable,  if  not  necessary,  to  employ  an  alter- 
nator giving  currents  which  rise  and  fall  harmonically,  as  in 
working  with  make  and  break  currents  the  observations  are  not 
always  trustworthy,  since  many  phenomena,  which  depend-  on 
the  rate  of  change,  may  be  produced  with  widely  different  fre- 
quencies. Even  when  making  such  observations  with  an  alternator 
one  is  apt  to  be  mistaken.  When  a  circuit  is  connected  to  an 
alternator  there  are  an  indefinite  number  of  values  for  capacity  and 
self-induction  which,  in  conjunction,  will  satisfy  the  condition  of 
resonance.  So  there  are  in  mechanics  an  infinite  number  of  tun- 
ing forks  which  will  respond  to  a  note  of  a  certain  pitch,  or  loaded 
springs  which  have  a  definite  period  of  vibration.  But  the  reson- 
ance will  be  most  perfectly  attained  in  that  case  in  which  the  mo- 
tion is  effected  with  the  greatest  freedom.  Now  in  mechanics, 
considering  the  vibration  in  the  common  medium — that  is,  air — it 
is  of  comparatively  little  importance  whether  one  tuning  fork  be 
somewhat  larger  than  another,  because  the  losses  in  the  air  are 
not  very  considerable.  One  may,  of  course,  enclose  a  tuning  fork 
in  an  exhausted  vessel  and  by  thus  reducing  the  air  resistance  to 
a  minimum  obtain  better  resonant  action.  Still  the  difference 
would  not  be  very  great.  But  it  would  make  a  great  difference  if 
the  tuning  fork  were  immersed  in  mercury.  In  the  electrical 
vibration  it  is  of  enormous  importance  to  arrange  the  conditions 
so  that  the  vibration  is  effected  with  the  greatest  freedom.  The 
magnitude  of  the  resonance  effect  depends,  under  otherwise  equal 
conditions,  on  the  quantity  of  electricity  set  in  motion  or  on  the 
strength  of  the  current  driven  through  the  circuit.  But  the  cir- 
cuit opposes  the  passage  of  the  currents  by  reason  of  its  imped- 
ance and  therefore,  to  secure  the  best  action  it  is  necessary  to  re- 
duce the  impedance  to  a  minimum.  It  is  impossible  to  overcome 
it  entirely,  but  merely  in  part,  for  the  ohmic  resistance  cannot  be 
overcame.  But  when  the  frequency  of  the  impulses  is  very  great, 
the  flow  of  the  current  is  practically  determined  by  self-induction. 
Now  self-induction  can  be  overcome  by  combining  it  with  capac- 
ity. If  the  relation  between  these  is  such,  that  at  the  frequency 
used  they  annul  each  other,  that  is,  have  such  values  as  to 
satisfy  the  condition  of  resonance,  and  the  greatest  quantity  of 
electricity  is  made  to  flow  through  the  external  circuit,  then  the 
best  result  is  obtained.  It  is  simpler  and  safer  to  join  the  con- 
denser in  series  with  the  self-induction.  It  is  clear  that  in  such 


844  INVENTIONS  OF  NIKOLA  TESLA. 

combinations  there  will  be,  for  a  given  frequency,  and  considering 
only  the  fundamental  vibration,  values  which  will  give  the  best 
result,  with  the  condenser  in  shunt  to  the  self-induction  coil ;  of 
course  more  such  values  than  with  the  condenser  in  series.  But 
practical  conditions  determine  the  selection.  In  the  latter  case 
in  performing  the  experiments  one  may  take  a  small  self-induction 
and  a  large  capacity  or  a  small  capacity  and  a  large  self-induc- 
tion, but  the  latter  is  preferable,  because  it  is  inconvenient  to  ad- 
just a  large  capacity  by  small  steps.  By  taking  a  coil  with  a  very 
large  self-induction  the  critical  capacity  is  reduced  to  a  very  small 
value,  and  the  capacity  of  the  coil  itself  may  be  sufficient.  It  is 
easy,  especially  by  observing  certain  artifices,  to  wind  a  coil 
through  which  the  impedance  will  be  reduced  to  the  value  of  the 
ohmic  resistance  only;  and  for  any  coil  there  is,  of  course,  a  fre- 
quency at  which  the  maximum  current  will  be  made  to  pass 
through  the  coil.  The  observation  of  the  relation  between  self- 


FIG.  184. 

induction,  capacity  and  frequency  is  becoming  important  in  the 
operation  of  alternate  current  apparatus,  such  as  transformers  or 
motors,  because  by  a  judicious  determination  of  the  elements  the 
employment  of  an  expensive  condenser  becomes  unnecessary. 
Thus  it  is  possible  to  pass  through  the  coils  of  an  alternating 
current  motor  under  the  normal  working  conditions  the  required 
current  with  a  low  E.  M.  F.  and  do  away  entirely  with  the  false 
current,  and  the  larger  the  motor,  the  easier  such  a  plan  becomes 
practicable  ;  but  it  is  necessary  for  this  to  employ  currents  of  very 
high  potential  or  high  frequency. 

In  Fig.  184  I.  is  shown  a  plan  which  has  been  followed  in  the 
study  of  the  resonance  effects  by  means  of  a  high  frequency  al- 
ternator. G!  is  a  coil  of  many  turns,  which  is  divided  into  small 
separate  sections  for  the  purpose  of  adjustment.  The  final  ad- 
justment was  made  sometimes  with  a  few  thin  iron  wires  (though 
this  is  not  always  advisable)  or  with  a  closed  secondary.  The  coil 


HIGH  FREQUENCY  AND  HIGH  POTENTIAL  CURRENTS.      345 

Cj  is  connected  with  one  of  its  ends  to  the  line  L  from  the  alter- 
nator G  and  with  the  other  end  to  one  of  the  plates  c  of  a  con- 
denser c  GI,  the  plate  (c^  of  the  latter  being  connected  to  a  much 
larger  plate  PJ.  In  this  manner  both  capacity  and  self-induction 
were  adjusted  to  suit  the  dynamo  frequency. 

As  regards  the  rise  of  potential  through  resonant  action,  of 
course,  theoretically,  it  may  amount  to  anything  since  it  depends 
on  self-induction  and  resistance  and  since  these  may  have  any 
value.  But  in  practice  one  is  limited  in  the  selection  of  these 
values  and  besides  these,  there  are  other  limiting  causes.  One 
may  start  with,  say,  1,000  volts  and  raise  the  E.  M.  F.  to  50  times 
that  value,  but  one  cannot  start  with  100,000  and  raise  it  to  ten 
times  that  value  because  of  the  losses  in  the  media  which  are 
great,  especially  if  the  frequency  is  high.  It  should  be  possible 
to  start  with,  for  instance,  two  volts  from  a  high  or  low  fre- 
quency circuit  of  a  dynamo  and  raise  the  E.  M.  r.  to  many  hun- 
dred times  that  value.  Thus  coils  of  the  proper  dimensions 
might  be  connected  each  with  only  one  of  its  ends  to  the 
mains  from  a  machine  of  low  E.  M.  F.,  and  though  the  circuit  of 
the  machine  would  not  be  closed  in  the  ordinary  acceptance  of  the 
term,  yet  the  machine  might  be  burned  out  if  a  proper  resonance 
effect  would  be  obtained.  I  have  not  been  able  to  produce,  nor 
have  I  observed  with  currents  from  a  dynamo  machine,  such 
great  rises  of  potential.  It  is  possible,  if  not  probable,  that  with 
currents  obtained  from  apparatus  containing  iron  the  disturbing 
influence  of  the  latter  is  the  cause  that  these  theoretical  pos- 
sibilities cannot  be  realized.  But  if  such  is  the  case  I  attribute 
it  solely  to  the  hysteresis  and  Foucault  current  losses  in  the  core. 
Generally  it  was  necessary  to  transform  upward,  when  the  E.  M. 
F.  was  very  low,  and  usually  an  ordinary  form  of  induction  coil 
was  employed,  but  sometimes  the  arrangement  illustrated  in  Fig. 
184  II.,  has  been  found  to  be  convenient.  In  this  case  a  coil  cis 
made  in  a  great  many  sections,  a  few  of  these  being  used  as  a 
primary.  In  this  manner  both  primary  and  secondary  are  ad- 
justable. One  end  of  the  coil  is  connected  to  the  line  i^  from 
the  alternator,  and  the  other  line  L  is  connected  to  the  intermedi- 
ate point  of  the  coil.  Such  a  coil  with  adjustable  primary  and 
secondary  will  be  found  also  convenient  in  experiments  with  the 
disruptive  discharge.  When  true  resonance  is  obtained  the  top 
of  the  wave  must  of  course  be  on  the  free  end  of  the  coil  as,  for 
instance,  at  the  terminal  of  the  phosphorescence  bulb  B.  This  is 


346  INVENTIONS  OF  NIKOLA  TESLA. 

easily  recognized  by  observing  the  potential  of  a  point  on  tl it- 
wire  w  near  to  the  coil. 

In  connection  with  resonance  effects  and  the  problem  of  trans- 
mission of  energy  over  a  single  conductor  which  was  previously 
considered,  I  would  say  a  few  words  on  a  subject  which  constantly 
fills  my  thoughts  and  which  concerns  the  welfare  of  all.  I  mean 
the  transmission  of  intelligible  signals  or  perhaps  even  power  to 
any  distance  without  the  use  of  wires.  I  am  becoming  daily 
more  convinced  of  the  practicability  of  the  scheme  ;  and  though 
I  know  full  well  that  the  great  majority  of  scientific  men  will 
not  believe  that  such  results  can  be  practically  and  immediately 
realized,  yet  I  think  that  all  consider  the  developments  in  recent 
years  by  a  number  of  workers  to  have  been  such  as  to  encourage 
thought  and  experiment  in  this  direction.  My  conviction  has 
grown  so  strong,  that  I  no  longer  look  upon  this  plan  of  energy 
or  intelligence  transmission  as  a  mere  theoretical  possibility,  but  as 
a  serious  problem  in  electrical  engineering,  which  must  be  carried 
out  some  day.  The  idea  of  transmitting  intelligence  without 
wires  is  the  natural  outcome  of  the  most  recent  results  of  elec- 
trical investigations.  Some  enthusiasts  have  expressed  their  be- 
lief that  telephony  to  any  distance  by  induction  through  the  air 
is  possible.  I  cannot  stretch  my  imagination  so  far,  but  I  do 
firmly  believe  that  it  is  practicable  to  disturb  by  means  of  power- 
ful machines  the  electrostatic  condition  of  the  earth  and  thus 
transmit  intelligible  signals  and  perhaps  power.  In  fact,  what  is 
there  against  the  carrying  out  of  such  a  scheme  ?  We  now  know 
that  electric  vibration  may  be  transmitted  through  a  single  con- 
ductor. Why  then  not  try  to  avail  ourselves  of  the  earth  for 
this  purpose  ?  We  need  not  be  frightened  by  the  idea  of  dis- 
tance. To  the  weary  wanderer  counting  the  mile-posts  the  earth 
may  appear  very  large,  but  to  that  happiest  of  all  men,  the  as- 
tronomer, who  gazes  at  the  heavens  and  by  their  standard  judges 
the  magnitude  of  our  globe,  it  appears  very  small.  And  so  I 
think  it  must  seem  to  the  electrician,  for  when  he  considers  the 
speed  with  which  an  electric  disturbance  is  propagated  through 
the  earth  all  his  ideas  of  distance  must  completely  vanish. 

A  point  of  great  importance  would  be  first  to  know  what  is  the 
capacity  of  the  earth  ?  and  what  charge  does  it  contain  if  electri- 
fied ?  Though  we  have  no  positive  evidence  of  a  charged  body 
existing  in  space  without  other  oppositely  electrified  bodies  being- 
near,  there  is  a  fair  probability  that  the  earth  is  such  a  body,  for 


HIGH  FREQUENCY  AND  HIGH  POTENTIAL  CURRENTS.     347 

by  whatever  process  it  was  separated  from  other  bodies — and  this 
is  the  accepted  view  of  its  origin — it  must  have  retained  a  charge, 
as  occurs  in  all  processes  of  mechanical  separation.  If  it  be  a 
charged  body  insulated  in  space  its  capacity  should  be  extremely 
small,  less  than  one-thousandth  of  a  farad.  But  the  upper  strata 
of  the  air  are  conducting,  and  so,  perhaps,  is  the  medium  in  free 
space  beyond  the  atmosphere,  and  these  may  contain  an  opposite 
charge.  Then  the  capacity  might  be  incomparably  greater.  In 
any  case  it  is  of  the  greatest  importance  to  get  an  idea  of  what 
quantity  of  electricity  the  earth  contains.  It  is  difficult  to  say 
whether  we  shall  ever  acquire  this  necessary  knowledge,  but  there 
is  hope  that  we  may,  and  that  is,  by  means  of  electrical  resonance. 
If  ever  we  can  ascertain  at  what  period  the  earth's  charge,  when 
disturbed,  oscillates  with  respect  to  an  oppositely  electrified  system 
or  known  circuit,  we  shall  know  a  fact  possibly  of  the  greatest 
importance  to  the  welfare  of  the  human  race.  I  propose  to  seek 
for  the  period  by  means  of  an  electrical  oscillator,  or  a  source  of 
alternating  electric  currents.  One  of  the  terminals  of  the  source 
would  be  connected  to  earth  as,  for  instance,  to  the  city  water 
mains*  the  other  to  an  insulated  body  of  large  surface.  It  is  pos- 
sible that  the  outer  conducting  air  strata,  or  free  space,  contain 
an  opposite  charge  and  that,  together  with  the  earth,  they  form  a 
condenser  of  very  large  capacity.  In  such  case  the  period  of 
vibration  may  be  very  low  and  an  alternating  dynamo  machine 
might  serve  for  the  purpose  of  the  experiment.  I  would  then 
transform  the  current  to  a  potential  as  high  as  it  would  be  found 
possible  and  connect  the  ends  of  the  high  tension  secondary  to  the 
ground  and  to  the  insulated  body.  By  varying  the  frequency  of  the 
currents  and  carefully  observing  the  potential  of  the  insulated  body 
and  watching  for  the  disturbance  at  various  neighboring  points  of 
the  earth's  surface  resonance  might  be  detected.  Should,  as  the 
majority  of  scientific  men  in  all  probability  believe,  the  period  be 
extremely  small,  then  a  dynamo  machine  would  not  do  and  a 
proper  electrical  oscillator  would  have  to  be  produced  and  perhaps 
it  might  not  be  possible  to  obtain  such  rapid  vibrations.  But 
whether  this  be  possible  or  not,  and  whether  the  earth  contains  a 
charge  or  not,  and  whatever  may  be  its  period  of  vibration,  it  cer- 
tainly is  possible — for  of  this  we  have  daily  evidence — to  pro- 
duce some  electrical  disturbance  sufficiently  powerful  to  be  per- 
ceptible by  suitable  instruments  at  any  point  of  the  earth's 
surface. 


348  INVENTIONS  OF  NIKOLA  TESLA. 

Assume  that  a  source  of  alternating  currentss  be  connected,  as 
in  Fig.  185,  with  one  of  its  terminals  to  earth  (conveniently  to  the 
water  mains)  and  with  the  other  to  a  body  of  large  surface  p. 
When  the  electric  oscillation  is  set  up  there  will  be 
a  movement  of  electricity  in  and  out  of  p,  and  alter- 
nating currents  will  pass  through  the  earth,  con- 
verging to,  or  diverging  from,  the  point  c  where 
the  ground  connection  is  made.  In  this  manner 
neighboring  points  on  the  earth's  surface  within  a 
certain  radius  will  be  disturbed.  But  the  distur- 
bance will  diminish  with  the  distance,  and  the  dis- 
tance at  which  the  effect  will  still  be  perceptible 
will  depend  on  the  quantity  of  electricity  set  in 
motion.  Since  the  body  p  is  insulated,  in  order  to 
displace  a  considerable  quantity,  the  potential  of 
the  source  must  be  excessive,  since  there  would  be 
limitations  as  to  the  surface  of  p.  The  conditions 
might  be  adjusted  so  that  the  generator  or  source 
s  will  set  up  the  same  electrical  movement  as 
though  its  circuit  were  closed.  Thus  it  is  certainly 
$2  practicable  to  impress  an  electric  vibration  at  least 
g  of  a  certain  low  period  upon  the  earth  by  means  of 
proper  machinery.  At  what  distance  such  a  vibra- 
tion might  be  made  perceptible  can  only  be  conjec- 
tured. I  have  on  another  occasion  considered  the 
question  how  the  earth  might  behave  to  electric 
disturbances.  There  is  no  doubt  that,  since  in  such 
an  experiment  the  electrical  density  at  the  surface 
could  be  but  extremely  small  considering  the  size 
of  the  earth,  the  air  would  not  act  as  a  very  dis- 
turbing factor,  and  there  would  be  not  much  energy 
lost  through  the  action  of  the  air,  which  would  be 
the  case  if  the  density  were  great.  Theoretically, 
then,  it  could  not  require  a  great  amount  of  energy 
to  produce  a  disturbance  perceptible  at  great  dis- 
tance, or  even  all  over  the  surface  of  the  globe. 
.  tq  Now,  it  is  quite  certain  that  at  any  point  within  a 

certain  radius  of  the  source  s  a  properly  adjusted 
self-induction  and  capacity  device  can  be  set  in  action 
by  resonance.  But  not  only  can  this  be  done,  but  another  source 
s,,  Fig.  185,  similar  to  s,  or  any  number  of  such  sources,  can  be  set 


HIGH  FREQUENCY  AND  HIGH  POTENTIAL  CURRENTS.     349 

to  work  in  synchronism  with  the  latter,  and  the  vibration  thus 
intensified  and  spread  over  a  large  area,  or  a  flow  of  elec- 
tricity produced  to  or  from  the  source  st  if  the  same  be  of 
opposite  phase  to  the  source  s.  I  think  that  beyond  doubt 
it  is  possible  to  operate  electrical  devices  in  a  city  through 
the  ground  or  pipe  system  by  resonance  from  an  electrical 
oscillator  located  at  a  central  point.  But  the  practical  solution 
of  this  problem  would  be  of  incomparably  smaller  benefit  to  man 
than  the  realization  of  the  scheme  of  transmitting  intelligence,  or 
perhaps  power,  to  any  distance  through  the  earth  or  environing 
medium.  If  this  is  at  all  possible,  distance  does  not  mean  any- 
thing. Proper  apparatus  must  first  be  produced  by  means  of 
which  the  problem  can  be  attacked  and  I  have  devoted  much 
thought  to  this  subject.  I  am  firmly  convinced  that  it  can  be 
done  and  hope  that  we  shall  live  to  see  it  done. 


ON  THE  LIGHT  PHENOMENA  PRODUCED  BY  HIGH-FREQUENCY  CUR- 
RENTS OF  HIGH  POTENTIAL  AND  GENERAL  REMARKS  RELATING 
TO  THE  SUBJECT. 

Returning  now  to  the  light  effects  which  it  has  been  the  chief 
object  to  investigate,  it  is  thought  proper  to  divide  these  effects 
into  four  classes  :  1.  Incandescence  of  a  solid.  2.  Phosphorescence. 

3.  Incandescence   or    phosphorescence    of   a  rarefied  gas ;  and 

4.  Luminosity  produced  in  a  gas  at  ordinary  pressure.     The  first 
question  is  :  How  are  these  luminous  effects  produced  ?     In  order 
to  answer  this  question  as  satisfactorily  as  I  am  able  to  do  in  the 
light  of  accepted  views  and  with  the  experience  acquired,  and  to 
add  some  interest  to  this  demonstration,  I  shall  dwell  here  upon 
a  feature  which  I  consider  of  great  importance,  inasmuch  as  it 
promises,  besides,  to  throw  a  better  light  upon  the  nature  of  most 
of  the  phenomena  produced  by  high-frequency  electric  currents. 
I  have  on  other  occasions  pointed  out  the  great  importance  of  the 
presence  of  the  rarefied  gas,  or  atomic  medium  in  general,  around 
the  conductor  through  which  alternate  currents  of  high  frequency 
are  passed,  as  regards  the  heating  of  the  conductor  by  the  cur- 
rents.    My  experiments,  described  some  time  ago,  have  shown 
that,  the  higher  the  frequency  and  potential  difference  of  the  cur- 
rents, the  more  important  becomes  the  rarefied  gas  in  which  the 
conductor  is  immersed,  as  a  factor  of  the  heating.     The  poten- 
tial difference,  however,  is,  as  I  then  pointed  out,  a  more  im- 


350  INVENTIONS  OF  NIKOLA  TESLA. 

portant  element  than  the  frequency.  When  both  of  these  are 
sufficiently  high,  the  heating  may  be  almost  entirely  due  to  the 
presence  of  the  rarefied  gas.  The  experiments  to  follow  will 
show  the  importance  of  the  rarefied  gas,  or,  generally,  of  gas  at  or- 
dinary or  other  pressure  as  regards  the  incandescence  or  other 
luminous  effects  produced  by  currents  of  this  kind. 

I  take  two  ordinary  50-volt  16  c.  p.  lamps  which  are  in  every 
respect  alike,  with  the  exception,  that  one  has  been  opened  at  the 
top  and  the  air  has  filled  the  bulb,  while  the  other  is  at  the  ordi- 
nary degree  of  exhaustion  of  commercial  lamps.  When  I  attach 
the  lamp  which  is  exhausted  to  the  terminal  of  the  secondary  of 
the  coil,  which  I  have  already  used,  as  in  experiments  illustrated 
in  Fig.  179«  for  instance,  and  turn  on  the  current,  the  filament,  as 
you  have  before  seen,  comes  to  high  incandescence.  When  I 
attach  the  second  lamp,  which  is  filled  with  air,  instead  of  the 
former,  the  filament  still  glows,  but  much  less  brightly.  This 
experiment  illustrates  only  in  part  the  truth  of  the  statements 
before  made.  The  importance  of  the  filament's  being  immersed 
in  rarefied  gas  is  plainly  noticeable  but  not  to  such  a  degree  as 
might  be  desirable.  The  reason  is  that  the  secondary  of  this  coil  is 
wound  for  low  tension,  having  only  150  turns,  and  the  potential 
difference  at  the  terminals  of  the  lamp  is  therefore  small.  Were 
I  to  take  another  coil  with  many  more  turns  in  the  secondary, 
the  effect  would  be  increased,  since  it  depends  partially  on  the 
potential  difference,  as  before  remarked.  But  since  the  effect 
likewise  depends  on  the  frequency,  it  may  be  properly  stated  that 
it  depends  on  the  time  rate  of  the  variation  of  the  potential  dif- 
ference. The  greater  this  variation,  the  more  important  becomes 
the  gas  as  an  element  of  heating.  I  can  produce  a  much  greater 
rate  of  variation  in  another  way,  which,  besides,  has  the  advan- 
tage of  doing  away  with  the  objections,  which  might  be  made  in 
the  experiment  just  shown,  even  if  both  the  lamps  were  con- 
nected in  series  or  multiple  arc  to  the  coil,  namely,  that  in  con- 
sequence of  the  reactions  existing  between  the  primary  and 
secondary  coil  the  conclusions  are  rendered  uncertain.  This  re- 
sult I  secure  by  charging,  from  an  ordinary  transformer  which  is 
fed  from  the  alternating  current  supply  station,  a  battery  of  con- 
densers, and  discharging  the  latter  directly  through  a  circuit  of 
small  self-induction,  as  before  illustrated  in  Figs.  183*,  183&, 
and  1836-. 

In  Figs.  186«,  1865  and  186c,  the  heavy  copper  bars  BB^  are 


HIGH  FREQUENCY  AND  HIGH  POTENTIAL  CURRENTS.     351 

connected  to  the  opposite  coatings  of  a  battery  of  condensers, 
or  generally  in  such  way,  that  the  high  frequency  or  sudden 
discharges  are  made  to  traverse  them.  I  connect  first  an 
ordinary  50-volt  incandescent  lamp  to  the  bars  by  means  of 
the  clamps  o  c.  The  discharges  being  passed  through  the  lamp, 
the  filament  is  rendered  incandescent,  though  the  current 
through  it  is  very  small,  and  would  not  be  nearly  sufficient  to 
produce  a  visible  effect  under  the  conditions  of  ordinary  use  of 
the  lamp.  Instead  of  this  I  now  attach  to  the  bars  another 
lamp  exactly  like  the  first,  but  with  the  seal  broken  off,  the  bulb 
being  therefore  filled  with  air  at  ordinary  pressure.  When  the 
discharges  are  directed  through  the  filament,  as  before,  it  does 
not  become  incandescent.  But  the  result  might  still  be  attri- 
buted to  one  of  the  many  possible  reactions.  I  therefore  connect 
both  the  lamps  in  multiple  arc  as  illustrated  in  Fig.  186«.  Passing 


FIG.  186a. 


FIG.  186b. 


FIG.  186c. 


the  discharges  through  both  the  lamps,  again  the  filament  in  the 
exhausted  lamp  I  glows  very  brightly  while  that  in  the  non-ex- 
hausted lamp  Zi  remains  dark,  as  previously.  But  it  should  not 
be  thought  that  the  latter  lamp  is  taking  only  a  small  fraction  of 
the  energy  supplied  to  both  the  lamps ;  on  the  contrary,  it  may 
consume  a  considerable  portion  of  the  energy  and  it  may  become 
even  hotter  than  the  one  which  burns  brightly.  In  this  experi- 
ment the  potential  difference  at  the  terminals  of  the  lamps  varies 
in  sign  theoretically  three  to  four  million  times  a  second.  The 
ends  of  the  filaments  are  correspondingly  electrified,  and  the  gas 
in  the  bulbs  is  violently  agitated  and  a  large  portion  of  the  sup- 
plied energy  is  thus  converted  into  heat.  In  the  non-exhausted 
bulb,  there  being  a  few  million  times  more  gas  molecules  than  in 
the  exhausted  one,  the  bombardment,  which  is  most  violent  at 
the  ends  of  the  filament,  in  the  neck  of  the  bulb,  consumes  a 


352  INVENTIONS  OF  NIKOLA  TESLA. 

large  portion  of  the  energy  without  producing  any  visible  effect. 
The  reason  is  that,  there  being  many  molecules,  the  bombard- 
ment is  quantitatively  considerable,  but  the  individual  impacts  are 
not  very  violent,  as  the  speeds  of  the  molecules  are  comparatively 
small  owing  to  the  small  free  path.  In  the  exhausted  bulb,  on 
the  contrary,  the  speeds  are  very  great,  and  the  individual  im- 
pacts are  violent  and  therefore  better  adapted  to  produce  a  visi- 
ble effect.  Besides,  the  convection  of  heat  is  greater  in  the  former 
bulb.  In  both  the  bulbs  the  current  traversing  the  filaments  is 
very  small,  incomparably  smaller  than  that  which  they  require  on 
an  ordinary  low-frequency  circuit.  The  potential  difference, 
however,  at  the  ends  of  the  filaments  is  very  great  and  might  be 
possibly  20,000  volts  or  more,  if  the  filaments  were  straight  and 
their  ends  far  apart.  In  the  ordinary  lamp  a  spark  generally  oc- 
curs between  the  ends  of  the  filament  or  between  the  platinum 
wires  outside,  before  such  a  difference  of  potential  can  be 
reached. 

It  might  be  objected  that  in  the  experiment  before  shown  the 
lamps,  being  in  multiple  arc,  the  exhausted  lamp  might  take  a 
much  larger  current  and  that  the  effect  observed  might  not  be 
exactly  attributable  to  the  action  of  the  gas  in  the  bulbs.  Such 
objections  will  lose  much  weight  if  I  connect  the  lamps  in  series, 
with  the  same  result.  When  this  is  done  and  the  discharges  are 
directed  through  the  filaments,  it  is  again  noted  that  the  filament 
in  the  non-exhausted  bulb  l^  remains  dark,  while  that  in  the 
exhausted  one  (7)  glows  even  more  intensely  than  under  its 
normal  conditions  of  working,  Fig.  1865.  According  to  general 
ideas  the  current  through  the  filaments  should  now  be  the  same, 
were  it  not  modified  by  the  presence  of  the  gas  around  the 
filaments. 

At  this  juncture  I  may  point  out  another  interesting  feature, 
which  illustrates  the  effect  of  the  rate  of  change  of  potential 
of  the  currents.  I  will  leave  the  two  lamps  connected  in  series 
to  the  bars  BB,,  as  in  the  previous  experiment,  Fig.  186&,  but  will 
presently  reduce  considerably  the  frequency  of  the  currents, 
which  was  excessive  in  the  experiment  just  before  shown.  This 
I  may  do  by  inserting  a  self-induction  coil  in  the  path  of  the  dis- 
charges, or  by  augmenting  the  capacity  of  the  condensers.  When 
I  now  pass  these  low-frequency  discharges  through  the  lamps, 
the  exhausted  lamp  I  again  is  as  bright  as  before,  but  it  is  noted 
also  that  the  non-exhausted  lamp  l±  glows,  though  not  quite 


HIGH  FJiKQ  UENCY  AND  HIGH  POTENTIAL  CURRENTS.      353 

as  intensely  as  the  other.  Reducing  the  current  through  the 
lamps,  I  may  bring  the  filament  in  the  latter  lamp  to  redness,  and, 
though  the  filament  in  the  exhausted  lamp  I  is  bright,  Fig.  I860, 
the  degree  of  its  incandescence  is  much  smaller  than  in  Fig.  1865, 
when  the  currents  were  of  a  much  higher  frequency. 

In  these  experiments  the  gas  acts  in  two  opposite  ways  in  de- 
termining the  degree  of  the  incandescence  of  the  filaments,  that 
is,  by  convection  and  bombardment.  The  higher  the  frequency  and 
potential  of  the  currents,  the  more  important  becomes  the  bom- 
bardment. The  convection  on  the  contrary  should  be  the  smaller, 
the  higher  the  frequency.  When  the  currents  are  steady  there  is 
practically  no  bombardment,  and  convection  may  therefore  with 
such  currents  also  considerably  modify  the  degree  of  incandescence 
and  produce  results  similar  to  those  just  before  shown.  Thus  if 
two  lamps  exactly  alike,  one  exhausted  and  one  not  exhausted, 
are  connected  in  multiple  arc  or  series  to  a  direct-current  machine, 
the  filament  in  the  non-exhausted  lamp  will  require  a  considera- 
bly greater  current  to  be  rendered  incandescent.  This  result  is 
entirely  due  to  convection,  and  the  effect  is  the  more  prominent 
the  thinner  the  filament.  Professor  Ayrton  and  Mr.  Kilgour 
some  time  ago  published  quantitative  results  concerning  the 
thermal  emissivity  by  radiation  and  convection  in  which  the  ef- 
fect with  thin  wires  was  clearly  shown.  This  effect  may  be  strik- 
ingly illustrated  by  preparing  a  number  of  small,  short,  glass  tubes, 
each  containing  through  its  axis  the  thinnest  obtainable  platinum 
wire.  If  these  tubes  be  highly  exhausted,  a  number  of  them 
may  be  connected  in  multiple  arc  to  a  direct-current  machine  and 
all  of  the  wires  may  be  kept  at  incandescence  with  a  smaller  cur- 
rent than  that  required  to  render  incandescent  a  single  one  of  the 
wires  if  the  tube  be  not  exhausted.  Could  the  tubes  be  so  highly 
exhausted  that  convection  would  be  nil,  then  the  relative  amounts 
of  heat  given  off  by  convection  and  radiation  could  be  deter- 
mined without  the  difficulties  attending  thermal  quantitative 
measurements.  If  a  source  of  electric  impulses  of  high  frequency 
and  very  high  potential  is  employed,  a  still  greater  number  of 
the  tubes  may  be  taken  and  the  wires  rendered  incandescent  by  a 
current  not  capable  of  warming  perceptibly  a  wire  of  the  same 
size  immersed  in  air  at  ordinary  pressure,  and  conveying  the 
energy  to  all  df  them. 

I  may  here  describe  a  result  which  is  still  more  interesting, 
and  to  which  I  have  been  led  by  the  observation  of  these  phe- 


354  INVENTIONS  OF  NIKOLA  TKSLA. 

nomena.  I  noted  that  small  differences  in  the  density  of  the  air 
produced  a  considerable  difference  in  the  degree  of  incandescence 
of  the  wires,  and  I  thought  that,  since  in  a  tube,  through  which 
a  luminous  discharge  is  passed,  the  gas  is  generally  not  of  uni- 
form density,  a  very  thin  wire  contained  in  the  tube  might  be 
rendered  incandescent  at  certain  places  of  smaller  density  of  the 
gas,  while  it  would  remain  dark  at  the  places  of  greater  density, 
where  the  convection  would  be  greater  and  the  bombardment  less 
intense.  Accordingly  a  tube  t  was  prepared,  as  illustrated  in  Fig. 
187,  which  contained  through  the  middle  a  very  line  platinum  wire 
w.  The  tube  was  exhausted  to  a  moderate  degree  and  it  was  found 
that  when  it  was  attached  to  the  terminal  of  a  high-frequency  coil 
the  platinum  wire  w  would  indeed,  become  incandescent  in  patches, 
as  illustrated  in  Fig.  187.  Later  a  number  of  these  tubes  with  one 
or  more  wires  were  prepared,  each  showing  this  result.  The  ef- 
fect was  best  noted  when  the  striated  discharge  occurred  in  the 
tube,  but  was  also  produced  when  the  stride  were  not  vi-ible, 
showing  that,  even  then,  the  gas  in  the  tube  was  not  of  uniform 
density.  The  position  of  the  strirp  was  generally  such,  that  the 
rarefactions  corresponded  to  the  places  of  incandescence  or  greater 
brightness  on  the  wire  w.  But  in  a  few  instances  it  was  noted,  that 
the  bright  spots  on  the  wire  were  covered  by  the  dense  parts  of 
the  striated  discharge  as  indicated  by  /  in  Fig.  187,  though  the  effect 
Avas  barely  perceptible.  This  was  explained  in  a  plausible  way 
by  assuming  that  the  convection  was  not  widely  different  in  the 
dense  and  rarefied  places,  and  that  the  bombardment  was  greater 
on  the  dense  places  of  the  striated  discharge.  It  is,  in  fact,  often 
observed  in  bulbs,  that  under  certain  conditions  a  thin  wire  is 
brought  to  higher  incandescence  when  the  air  is  not  too  highly 
rarefied.  This  is  the  case  when  the  potential  of  the  coil  is  not 
high  enough  for  the  vacuum,  but  the  result  may  be  attributed  to 
many  different  causes.  In  all  cases  this  curious  phenomenon  of 
incandescence  disappears  when  the  tube,  or  rather  the  wire, 
.acquires  throughout  a  uniform  temperature. 

Disregarding  now  the  modifying  effect  of  convection  there  are 
then  two  distinct  causes  which  determine  the  incandescence  of  a 
wire  or  filament  with  varying  currents,  that  is,  conduction  cur- 
rent and  bombardment.  With  steady  currents  we  have  to  deal 
only  with  the  former  of  these  two  causes,  and  the  heating  effect 
is  a  minimum,  since  the  resistance  is  least  to  steady  fiow.  When 
the  current  is  a  varying  one  the  resistance  is  greater,  and  hence 


HIGH  FREQ  UWCY  AND  HIGH  POTENTIA  L  CURRENTS.      355 


the  heating  effect  is  increased.  Thus  if  the  rate  of  change  of 
the  current  is  very  great,  the  resistance  may  increase  to  such 
an  extent  that  the  filament  is  brought  to  incandescence  with  in- 
appreciable currents,  and  we  are  able  to  take  a  short  and  thick 
block  of  carbon  or  other  material  and  bring  it  to  bright  incan- 
descence with  a  current  incomparably  smaller  than  that  required 
to  bring  to  the  same  degree  of  incandescence  an  ordinary  thin 
lamp  filament  with  a  steady  or  low  frequency  current.  This  result 
is  important,  and  illustrates  how  rapidly  our  views  on  these  sub- 
jects are  changing,  and  how  quickly  our  field  of  knowledge  is  ex- 


FIG.  187. 


FIG.  188. 


tending.  In  the  art  of  incandescent  lighting,  to  view  this  result 
in  one  aspect  only,  it  has  been  commonly  considered  as  an  essen- 
tial requirement  for  practical  success,  that  the  lamp  filament 
should  be  thin  and  of  high  resistance.  But  now  we  know  that 
the  resistance  of  the  filament  to  the  steady  flow  does  not  mean 
anything  ;  the  filament  might  as  well  be  short  and  thick  ;  for  if  it 
be  immersed  in  rareiied  gas  it  will  become  incandescent  by  the 
passage  of  a  small  current.  It  all  depends  on  the  frequency  and 
potential  of  the  currents.  We  may  conclude  from  this,  that  it 


356  INVENTIONS  OF  NIKOLA  TESLA. 

would  be  of  advantage,  so  far  as  the  lamp  is  considered,  to  em- 
ploy high  frequencies  for  lighting,  as  they  allow  the  use  of  short 
and  thick  filaments  and  smaller  currents. 

If  a  wire  or  filament  be  immersed  in  a  homogeneous  medium,  all 
the  heating  is  due  to  true  conduction  current,  but  if  it  be  enclosed 
in  an  exhausted  vessel  the  conditions  are  entirely  different.  Here 
the  gas  begins  to  act  and  the  heating  effect  of  the  conduction  cur- 
rent, as  is  shown  in  many  experiments,  may  be  very  small  com- 
pared with  that  of  the  bombardment.  This  is  especially  the  case  if 
the  circuit  is  not  closed  and  the  potentials  are  of  course  very  high. 
Suppose  that  a  fine  filament  enclosed  in  an  exhausted  vessel  be 
connected  with  one  of  its  ends  to  the  terminal  of  a  high  tension 
coil  and  with  its  other  end  to  a  large  insulated  plate.  Though 
the  circuit  is  not  closed,  the  filament,  as  I  have  before  shown,  is 
brought  to  incandescence.  If  the  frequency  and  potential  be 
comparatively  low,  the  filament  is  heated  by  the  current  passing 
through  it.  If  the  frequency  and  potential,  and  principally  the 
latter,  be  increased,  the  insulated  plate  need  'be  but  very  small,  or 
may  be  done  away  with  entirely  ;  still  the  filament  will  become 
incandescent,  practically  all  the  heating  being  then  due  to  the  bom- 
bardment. A  practical  way  of  combining  both  the  effects  of 
conduction  currents  and  bombardment  is  illustrated  in  Fig.  188, 
in  which  an  ordinary  lamp  is  shown  provided  with  a  very  thin 
filament  which  has  one  of  the  ends  of  the  latter  connected  to  a 
shade  serving  the  purpose  of  the  insulated  plate,  and  the  other 
end  to  the  terminal  of  a  high  tension  source.  It  should  not  be 
thought  that  only  rarefied  gas  is  an  important  factor  in  the  heat- 
ing of  a  conductor  by  varying  currents,  but  gas  at  ordinary  pres- 
sure may  become  important,  if  the  potential  difference  and  fre- 
quency of  the  currents  is  excessive.  On  this  subject  I  have  al- 
ready stated,  that  when  a  conductor  is  fused  by  a  stroke  of 
lightning,  the  current  through  it  may  be  exceedingly  small,  not 
even  sufficient  to  heat  the  conductor  perceptibly,  were  the  latter 
immersed  in  a  homogeneous  medium. 

From  the  preceding  it  is  clear  that  when  a  conductor  of  high 
resistance  is  connected  to  the  terminals  of  a  source  of  high  fre- 
quency currents  of  high  potential,  there  may  occur  considerable 
dissipation  of  energy,  principally  at  the  ends  of  the  conductor,  in 
consequence  of  the  action  of  the  gas  surrounding  the  conductor. 
Owing  to  this,  the  current  through  a  section  of  the  conductor  at 
a  point  midway  between  its  ends  may  be  much  smaller  than 


HIGH  FRE^JENOY  AND  HIGH  POTENTIAL  CURRENTS.     357 

through  a  section  near  the  ends.  Furthermore,  the  current  passes 
principally  through  the  outer  portions  of  the  conductor,  but  this 
effect  is  to  be  distinguished  from  the  skin  effect  as  ordinarily  in- 
terpreted, for  the  latter  would,  or  should,  occur  also  in  a  continu- 
ous incompressible  medium.  If  a  great  many  incandescent  lamps 
are  connected  in  series  to  a  source  of  such  currents,  the  lamps  at 
the  ends  may  burn  brightly,  whereas  those  in  the  middle  may  re- 
main entirely  dark.  This  is  due  principally  to  bombardment,  as 
before  stated.  But  even  if  the  currents  be  steady,  provided  the 
difference  of  potential  is  very  great,  the  lamps  at  the  end  will 
burn  more  brightly  than  those  in  the  middle.  In  such  case  there 
is  no  rhythmical  bombardment,  and  the  result  is  •  produced  en- 
tirely by  leakage.  This  leakage  or  dissipation  into  space  when 
the  tension  is  high,  is  considerable  when  incandescent  lamps  are 
used,  and  still  more  considerable  with  arcs,  for  the  latter  act  like 
flames.  Generally,  of  course,  the  dissipation  is  much  smaller 
with  steady,  than  with  varying,  currents. 

I  have  contrived  an  experiment  which  illustrates  in  an  inter- 
esting manner  the  effect  of  lateral  diffusion.  If  a  very  long  tube 
is  attached  to  the  terminal  of  a  high  frequency  coil,  the  luminos- 
ity is  greatest  near  the  terminal  and  falls  off  gradually  towards 
the  remote  end.  This  is  more  marked  if  the  tube  is  narrow. 

A  small  tube  about  one-half  inch  in  diameter  and  twelve 
inches  long  (Fig.  189),  has  one  of  its  ends  drawn  out  into  a  fine 
fibre/  nearly  three  feet  long.  The  tube  is  placed  in  a  brass  socket 
T  which  can  be  screwed  on  the  terminal  TX  of  the  induction  coil. 
The  discharge  passing  through  the  tube  first  illuminates  the  bot- 
tom of  the  same,  which  is  of  comparatively  large  section ;  but 
through  the  long  glass  fibre  the  discharge  cannot  pass.  But 
gradually  the  rarefied  gas  inside  becomes  warmed  and  more  con- 
ducting and  the  discharge  spreads  into  the  glass  fibre.  This  spread- 
ing is  so  slow,  that  it  may  take  half  a  minute  or  more  until  the 
discharge  has  worked  through  up  to  the  top  of  the  glass  fibre, 
then  presenting  the  appearance  of  a  strongly  luminous  thin 
thread.  By  adjusting  the  potential  at  the  terminal  the  light  may 
be  made  to  travel  upwards  at  any  speed.  Once,  however,  the 
glass  fibre  is  heated,  the  discharge  breaks  through  its  entire 
length  instantly.  The  interesting  point  to  be  noted  is  that,  the 
higher  the  frequency  of  the  currents,  or  in  other  words,  the 
greater  relatively  the  lateral  dissipation,  at  a  slower  rate  may  the 
light  be  made  to  propagate  through  the  fibre.  This  experiment 


358  INVENTIONS  OF  NIKOLA  TE8L*.  ~ 

\ 

is  best  performed  with  a  highly  exhausted  and  freshly  made  tube. 
When  the  tube  has  been  used  for  some  time  the  experiment 
often  fails.  It  is  possible  that  the  gradual  and  slow  impairment 
of  the  vacuum  is  the  cause.  This  slow  propagation  of  the  dis- 
charge through  a  very  narrow  glass  tube  corresponds  exactly  to 
the  propagation  of  heat  through  a  bar  warmed  at  one  end.  The 
quicker  the  heat  is  carried  away  laterally  the  longer  time  it  will 
take  for  the  heat  to  warm  the  remote  end.  When  the  current 
of  a  low  frequency  coil  is  passed  through  the  fibre  from  end  to 
end,  then  the  lateral  dissipation  is  small  and  the  discharge  in- 
stantly breaks  through  almost  without  exception. 


FIG.  189. 


FIG.  l»0. 


•After  these  experiments  and  observations  which  have  shown 
the  importance  of  the  discontinuity  or  atomic  structure  of  the 
medium  and  which  will  serve  to  explain,  in  a  measure  at  least, 
the  nature  of  the  four  kinds  of  light  effects  producible  with 
these  currents,  I  may  now  give  you  an  illustration  of  these 
effects.  For  the  sake  of  interest  I  may  do  this  in  a  manner 
which  to  many  of  you  might  be  novel.  You  have  seen  before 
that  we  may  now  convey  the  electric  vibration  to  a  body  by 
means  of  a  single  wire  or  conductor  of  any  kind.  Since  the 


HIGH  FREQ  UENCY  AND  HIGH  FOTENTIA  L  C URRENTS.      359 


human  frame  is  conducting  I  may  convey  the  vibration  through 
my  body. 

First,  as  in  some  previous  experiments,  I  connect  my  body  with 
one  of  the  terminals  of  a  high-tension  transformer  and  take  in  my 
hand  an  exhausted  bulb  which  contains  a  small  carbon  button 
mounted  upon  a  platinum  wire  leading  to  the  outside  of  the  bulb, 
and  the  button  is  rendered  incandescent  as  soon  as  the  transformer 
is  set  to  work  (Fig.  190).  I  may  place  a  conducting  shade  on  the 
bulb  which  serves  to  intensify  the  action,  but  is  not  necessary. 
Nor  is  it  required  that  the  button  should  be  in  conducting  con- 
nection witli  the  hand  through  a  wire  leading  through  the  glass, 


FIG.  192. 

for  sufficient  energy  may  be  transmitted  through  the  glass  itself 
by  inductive  action  to  render  the  button  incandescent. 

Next  I  take  a  highly  exhausted  bulb  containing  a  strongly 
phosphorescent  body,  above  which  is  mounted  a  small  plate  of 
aluminum  on  a  platinum  wire  leading  to  the  outside,  and  the  cur- 
rents flowing  through  my  body  excite  intense  phosphorescence 
in  the  bulb  (Fig.  191).  Next  again  I  take  in  my  hand  a  simple 
exhausted  tube,  and  in  the  same  manner  the  gas  inside  the  tube 
is  rendered  highly  incandescent  or  phosphorescent  (Fig.  192). 
Finally,  I  may  take  in  my  hand  a  wire,  bare  or  covered  with  thick 
insulation,  it  is  quite  immaterial;  the  electrical  vibration  is  su 
intense  as  to  cover  the  wire  with  a  luminous  film  (Fig.  193). 


360  INVENTIONS  OF  NIKOLA  TESLA. 

A  few  words  must  now  be  devoted  to  each  of  these  phenomena. 
In  the  first  place,  I  will  consider  the  incandescence  of  a  button  or  of 
a  solid  in  general,  and  dwell  upon  some  facts  which  apply  equally 
to  all  these  phenomena.  It  was  pointed  out  before  that  when  a 
thin  conductor,  such  as  a  lamp  filament,  for  instance,  is  connected 
with  one  of  its  ends  to  the  terminal  of  a  transformer  of  high 
tension  the  filament  is  brought  to  incandescence  partly  by  a 
conduction  current  and  partly  by  bombardment.  The  shorter 
and  thicker  the  filament  the  more  important  becomes  the  latter, 
and  finally,  reducing  the  filament  to  a  mere  button,  all  the  heat- 
ing must  practically  be  attributed  to  the  bombardment.  So  in 
the  experiment  before  shown,  the  button  is  rendered  incandescent 
by  the  rhythmical  impact  of  freely  movable  small  bodies  in  the 
bulb.  These  bodies  may  be  the  molecules  of  the  residual  gas, 
particles  of  dust  or  lumps  torn  from  the  electrode  ;  whatever  they 
are,  it  is  certain  that  the  heating  of  the  button  is  essentially  con- 
nected with  the  pressure  of  such  freely  movable  particles,  or  of 
atomic  matter  in  general  in  the  bulb.  The  heating  is  the  more 
intense  the  greater  the  number  of  impacts  per  second  and  the 
greater  the  energy  of  each  impact.  Yet  the  button  would 
be  heated  also  if  it  were  connected  to  a  source  of  a  steady  po- 
tential. In  such  a  case  electricity  would  be  carried  away  from 
the  button  by  the  freely  movable  carriers  or  particles  flying 
about,  and  the  quantity  of  electricity  thus  carried  away  might  be 
sufficient  to  bring  the  button  to  incandescence  by  its  passage 
through  the  latter.  But  the  bombardment  could  not  be  of  great 
importance  in  such  case.  For  this  reason  it  would  require  a  com- 
paratively very  great  supply  of  energy  to  the  button  to  maintain 
it  at  incandescence  with  a  steady  potential.  The  higher  the  fre- 
quency of  the  electric  impulses  the  more  economically  can  the 
button  be  maintained  at  incandescence.  One  of  the  chief  rea- 
sons why  this  is  so,  is,  I  believe,  that  with  impulses  of  very  high 
frequency  there  is  less  exchange  of  the  freely  movable  carriers 
around  the  electrode  and  this  means,  that  in  the  bulb  the  heated 
matter  is  better  confined  to  the  neighborhood  of  the  button.  If 
a  double  bulb,  as  illustrated  in  Fig.  194  be  made,  comprising  a 
large  globe  B  and  a  small  one  5,  each  containing  as  usual  a  fila- 
ment/" mounted  on  a  platinum  wire  w  and  wt,  it  is  found,  that  if 
the  filaments  ff  be  exactly  alike,  it  requires  less  energy  to  keep 
the  filament  in  the  globe  b  at  a  certain  degree  of  incandescence, 
than  that  in  the  globe  B.  This  is  due  to  the  confinement  of  the 


HIGH  FREQUENCY  AND  HIGH  POTENTIAL  CURRENTS      361 

movable  particles  around  the  button.  In  this  case  it  is  also  ascer- 
tained, that  the  filament  in  the  small  globe  5  is  less  deteriorated 
when  maintained  a  certain  length  of  time  at  incandescence.  This 
is  a  necessary  consequence  of  the  fact  that  the  gas  in  the  small 
bulb  becomes  strongly  heated  and  therefore  a  very  good  con- 
ductor, and  less  work  is  then  performed  on  the  button,  since  the 
bombardment  becomes  less  intense  as  the  conductivity  of  the  gas 
increases.  In  this  construction,  of  course,  the  small  bulb  becomes 
very  hot  and  when  it  reaches  an  elevated  temperature  the  con- 
vection and  radiation  on  the  outside  increase.  On  another  oc- 
casion I  have  shown  bulbs  in  which  this  drawback  was  largely 
avoided.  In  these  instances  a  very  small  bulb,  containing  a  re- 
fractory button,  was  mounted  in  a  large  globe  and  the  space  be- 


FIG.  193. 


FIG.  194. 


tween  the  walls  of  both  was  highly  exhausted.  The  outer  large 
globe  remained  comparatively  cool  in  such  constructions.  When 
the  large  globe  was  on  the  pump  and  the  vacuum  between  the 
walls  maintained  permanent  by  the  continuous  action  of  the 
pump,  the  outer  globe  would  remain  quite  cold,  while  the  button 
in  the  small  bulb  was  kept  at  incandescence.  But  when  the  seal 
was  made,  and  the  button  in  the  small  bulb  maintained  incan- 
descent some  length  of  time,  the  large  globe  too  would  become 
warmed.  From  this  I  conjecture  that  if  vacuous  space  (as  Prof. 
Dewar  finds)  cannot  convey  heat,  it  is  so  merely  in  virtue  of  our 
rapid  motion  through  space  or,  generally  speaking,  by  the  moti  n 
of  the  medium  relatively  to  us,  for  a  permanent  condition  could 


362  INVENTIONS  OF  NIKOLA  TESLA. 

not  be  maintained  without  the  medium  being  constantly  renewed. 
A  vacuum  cannot,  according  to  all  evidence,  be  permanently 
maintained  around  a  hot  body. 

In  these  constructions,  before  mentioned,  the  small  bulb  inside 
would,  at  least  in  the  first  stages,  prevent  all  bombardment 
a*  -  against  the  outer  large  globe.  It  occurred  to  me  then  to  ascer- 
tain how  a  metal  sieve  would  behave  in  this  respect,  and  several 
bulbs,  as  illustrated  in  Fig.  195,  were  prepared  for  this  purpose. 
r .  In  a  globe  &,  was  mounted  a  thin  filament  f  (or  button)  upon  a 
platinum  wire  w  passing  through  a  glass  stem  and  leading  to  the 
outside  of  the  globe.  The  filament /"was  surrounded  by  a  metal 
sieve  s.  It  was  found  in  experiments  with  such  bulbs  that  a  sieve 
with  wide  meshes  apparently  did  not  in  the  slightest  affect  the 
bombardment  against  the  globe  b.  When  the  vacuum  was  high, 
the  shadow  of  the  sieve  was  clearly  projected  against  the  globe 
and  the  latter  would  get  hot  in  a  short  while.  In  some  bulbs  the 
sieve  .s  was  connected  to  a  platinum  wire  sealed  in  the  glass. 
When  this  wrire  was  connected  to  the  other  terminal  of  the  induc- 
tion coil  (the  E.  M.  F.  being  kept  low  in  this  case),  or  to  an  insu- 
lated plate,  the  bombardment  against  the  outer  globe  1)  was 
diminished.  By  taking  a  sieve  with  fine  meshes  the  bombard- 
ment against  the  globe  b  was  always  diminished,  but  even  then 
if  the  exhaustion  was  carried  very  far,  and  when  the  potential  of 
the  transformer  was  very  high,  the  globe  1}  would  be  bombarded 
and  heated  quickly,  though  no  shadow  pf  the  sieve  was  visible, 
owing  to  the  smallness  of  the  meshes.  But  a  glass  tube  or  other 
continuous  body  mounted  so  as  to  surround  the  filament,  did  en- 
tirely cut  off  the  bombardment  and  for  a  while  the  outer  globe  b 
would  remain  perfectly  cold.  Of  course  when  the  glass  tube 
was  sufficiently  heated  the  bombardment  against  the  outer  globe 
could  be  noted  at  once.  The  experiments  with  these  bulbs 
seemed  to  show  that  the  speeds  of  the  projected  molecules  or 
particles  must  be  considerable  (though  quite  insignificant  when 
compared  with  that  of  light),  otherwise  it  would  be  difficult  to 
understand  how  they  could  traverse  a  fine  metal  sieve  without 
being  affected,  unless  it  were  found  that  such  small  particles  or 
atoms  cannot  be  acted  upon  directly  at  measurable  distances. 
In  regard  to  the  speed  of  the  projected  atoms,  Lord  Kelvin  has 
recently  estimated  it  at  about  one  kilometre  a  second  or  there- 
abouts in  an  ordinary  Crookes  bulb.  As  the  potentials  obtainable 
with  a  disruptive  discharge  coil  are  much  higher  than  with  or- 


HIGH  FREQUENCY  AND  HIGH  POTENTIAL  CURRENTS.      :',(>:; 

dinary  coils,  the  speeds  must,  of  course,  be  much  greater  when 
the  bulbs  are  lighted  from  such  a  coil.  Assuming  the  speed  to 
be  as  high  as  five  kilometres  and  uniform  through  the  whole 
trajectory,  as  it  should  be  in  a  very  highly  exhausted  vessel,  then 
if  the  alternate  electrifications  of  the  electrode  would  be  of  a 
frequency  of  five  million,  the  greatest  distance  a  particle  could 
get  away  from  the  electrode  would  be  one  millimetre,  and  if  it 
could  be  acted  upon  directly  at  that  distance,  the  exchange  of 
electrode  matter  or  of  the  atoms  would  be  very  slow  and  there 
would  be  practically  no  bombardment  against  the  bulb.  This  at 
least  should  be  so,  if  the  action  of  an  electrode  upon  the  atoms 
of  the  residual  gas  would  be  such  as  upon  electrified  bodies  which 
we  can  perceive.  A  hot  body  enclosed  in  an  exhausted  bulb 
produces  always  atomic  bombardment,  but  a  hot  body  has  no 
definite  rhythm,  for  its  molecules  perform  vibrations  of  all  kinds. 

If  a  bulb  containing  a  button  or  filament  be  exhausted  as  high 
as  is  possible  with  the  greatest  care  and  by  the  use  of  the  best  ar- 
tifices, it  is  often  observed  that  the  discharge  cannot,  at  first, 
break  through,  but  after  some  time,  probably  in  consequence  of 
some  changes  within  the  bulb,  the  discharge  finally  passes  through 
and  the  button  is  rendered  incandescent.  In  fact,  it  appears  that 
the  higher  the  degree  of  exhaustion  the  easier  is  the  incandescence 
produced.  There  seem  to  be  no  other  causes  to  which  the  in- 
candescence might  be  attributed  in  such  case  except  to  the  bom- 
bardment or  similar  action  of  the  residual  gas,  or  of  particles  of 
matter  in  general.  But  if  the  bulb  be  exhausted  with  the  great- 
est care  can  these  play  an  important  part  ?  Assume  the  vacuum 
in  the  bulb  to  be  tolerably  perfect,  the  great  interest  then  centres 
in  the  question :  Is  the  medium  which  pervades  all  space  con- 
tinuous or  atomic  ?  If  atomic,  then  the  heating  of  a  conducting 
button  or  filament  in  an  exhausted  vessel  might  be  due  largely 
to  ether  bombardment,  and  then  the  heating  of  a  conductor  in 
general  through  which  currents  of  high  frequency  or  high  poten- 
tial are  passed  must  be  modified  by  the  behavior  of  such  medium  ; 
then  also  the  skin  effect,  the  apparent  increase  of  the  ohmic  re- 
sistance, etc.,  admit,  partially  at  least,  of  a  different  explanation. 

It  is  certainly  more  in  accordance  with  many  phenomena  ob- 
served with  high-frequency  currents  to  hold  that  all  space  is  per- 
vaded with  free  atoms,  rather  than  to  assume  that  it  is  devoid  of 
these,  and  dark  and  cold,  for  so  it  must  be,  if  filled  with  a  con- 
tinuous medium,  since  in  such  there  can  be  neither  heat  nor  light. 


364  INVENTIONS  OF  NIKOLA  TE8LA. 

Is  then  energy  transmitted  by  independent  carriers  or  by  the 
vibration  of  a  continuous  medium  ?  This  important  question  is 
by  no  means  as  yet  positively  answered.  But  most  of  the  effects 
which  are  here  considered,  especially  the  light  effects,  incandes- 
cence, or  phosphorescence,  involve  the  presence  of  free  atoms  and 
would  be  impossible  without  these. 

In  regard  to  the  incandescence  of  a  refractory  button  (or  fila- 
ment) in  an  exhausted  receiver,  which  has  been  one  of  the  sub- 
jects of  this  investigation,  the  chief  experiences,  which  may  serve 
as  a  guide  in  constructing  such  bulbs,  may  be  summed  up  as  fol- 
lows :  1.  The  button  should  be  as  small  as  possible,  spherical, 
of  a  smooth  or  polished  surface,  and  of  refractory  material  which 
withstands  evaporation  best.  2.  The  support  of  the  button 
should  be  very  thin  and  screened  by  an  aluminum  and  mica  sheet, 
as  I  have  described  on  another  occasion.  3.  The  exhaustion  of 
the  bulb  should  be  as  high  as  possible.  4.  The  frequency  of  the 
currents  should  be  as  high  as  practicable.  5.  The  currents  should 
be  of  a  harmonic  rise  and  fall,  without  sudden  interruptions.  6. 
The  heat  should  be  confined  to  the  button  by  inclosing  the  same 
in  a  small  bulb  or  otherwise.  7.  The  space  between  the  walls  of 
the  small  bulb  and  the  outer  globe  should  be  highly  exhausted. 
Most  of  the  considerations  which  apply  to  the  incandescence 
of  a  solid  just  considered  may  likewise  be  applied  to  phosphor- 
escence. Indeed,  in  an  exhausted  vessel  the  phosphorescence  is, 
as  a  rule,  primarily  excited  by  the  powerful  beating  of  the  elec- 
trode stream  of  atoms  against  the  phosphorescent  body.  Even  in 
many  cases,  where  there  is  no  evidence  of  such  a  bombardment, 
I  think  that  phosphorescence  is  excited  by  violent  impacts  of 
atoms,  which  are  not  necessarily  thrown  off  from  the  electrode 
but  are  acted  upon  from  the  same  inductively  through  the 
medium  or  through  chains  of  other  atoms.  That  mechanical 
shocks  play  an  important  part  in  exciting  phosphorescence  in  a 
bulb  may  be  seen  from  the  following  experiment.  If  a  bulb, 
constructed  as  that  illustrated  in  Fig.  1Y4,  be  taken  and  exhausted 
with  the  greatest  care  so  that  the  discharge  cannot  pass,  the  fila- 
ment f  acts  by  electrostatic  induction  upon  the  tube  t  and  the 
latter  is  set  in  vibration.  If  the  tube  o  be  rather  wide,  about  an 
inch  or  so,  the  filament  may  be  so  powerfully  vibrated  that  when- 
ever it  hits  the  glass  tube  it  excites  phosphorescence.  But  the 
phosphorescence  ceases  when  the  filament  comes  to  rest.  The 
vibration  can  be  arrested  and  again  started  by  varying  the 


HIGH  FEEQ  UENCT  AND  HIGH  POTENTIAL  CURRENTS.      365 

frequency  of  the  currents.  Now  the  filament  lias  its  own 
period  of  vibration,  and  if  the  frequency  of  the  currents  is  such 
that  there  is  resonance,  it  is  easily  set  vibrating,  though  the  po- 
tential of  the  currents  be  small.  I  have  often  observed  that  the 
filament  in  the  bulb  is  destroyed  by  such  mechanical  resonance. 
The  filament  vibrates  as  a  rule  so  rapidly  that  it  cannot  be  seen 
and  the  experimenter  may  at  first  be  mystified.  When  such  an 
experiment  as  the  one  described  is  carefully  performed,  the  po- 
tential of  the  currents  need  be  extremely  small,  and  for  this 
reason  I  infer  that  the  phosphorescence  is  then  due  to  the 
mechanical  shock  of  the  filament  against  the  glass,  just  as  it  is 
produced  by  striking  a  loaf  of  sugar  with  a  knife.  The  mechani- 
cal shock  produced  by  the  projected  atoms  is  easily  noted  when 
a  bulb  containing  a  button  is  grasped  in  the  hand  and  the  cur- 
rent turned  on  suddenly.  I  believe  that  a  bulb  could  be  shat- 
tered by  observing  the  conditions  of  resonance. 

In  tlie  experiment  before  cited  it  is,  of  course,  open  to  say, 
that  the  glass  tube,  upon  coming  in  contact  with  the  filament,  re- 
tains a  charge  of  a  certain  sign  upon  the  point  of  contact.  If 
now  the  filament  again  touches  the  glass  at  the  same  point  while 
it  is  oppositely  charged,  the  charges  equalize  under  evolution  of 
light.  But  nothing  of  importance  would  be  gained  by  such  an 
explanation.  It  is  unquestionable  that  the  initial  charges  given 
to  the  atoms  or  to  the  glass  play  some  part  in  exciting  phospho- 
rescence. So,  for  instance,  if  a  phosphorescent  bulb  be  first  ex- 
cited by  a  high  frequency  coil  by  connecting  it  to  one  of  the  ter- 
minals of  the  latter  and  the  degree  of  luminosity  be  noted,  and  then 
the  bulb  be  highly  charged  from  a  Holtz  machine  by  attaching 
it  preferably  to  the  positive  terminal  of  the  machine,  it  is  found 
that  when  the  bulb  is  again  connected  to  the  terminal  of  the  high 
frequency  coil,  the  phosphorescence  is  far  more  intense.  On 
another  occasion  I  have  considered  the  possibility  of  some  phos- 
phorescent phenomena  in  bulbs  being  produced  by  the  incandes- 
cence of  an  infinitesimal  layer  on  the  surface  of  the  phosphores- 
cent body.  Certainly  the  impact  of  the  atoms  is  powerful  enough 
to  produce  intense  incandescence  by  the  collisions,  since  they  bring 
quickly  to  a  high  temperature  a  body  of  considerable  bulk.  If  any 
such  effect  exists,  then  the  best  appliance  for  producing  phospho- 
rescence in  a  bulb,  which  we  know  so  far,  is  a  disruptive  discharge 
coil  giving  an  enormous  potential  with  but  few  fundamental  dis- 
charges, say  25-30  per  second,  just  enough  to  produce  a  continu- 


366  INVENTIONS  OF  NIKOLA  TES1.A. 

ous  impression  upon  the  eye.  It  is  a  fact  that  such  a  coil  excites 
phosphorescence  under  almost  any  condition  and  at  all  degrees 
of  exhaustion,  and  I  have  observed  effects  which  appear  to  be  due 
to  phosphorescence  even  at  ordinary  pressures  of  the  atmosphere, 
when  the  potentials  are  extremely  high.  But  if  phosphorescent 
light  is  produced  by  the  equalization  of  charges  of  electrified 
atoms  (whatever  this  may  mean  ultimately),  then  the  higher  the 
frequency  of  the  impulses  or  alternate  electrifications,  the 
more  economical  will  be  the  light  production.  It  is  a  long 
known  and  noteworthy  fact  that  all  the  phosphorescent  bodies 
are  poor  conductors  of  electricity  and  heat,  and  that  all  bodies 
cease  to  emit  phosphorescent  light  when  they  are  brought  to  a 
certain  temperature.  Conductors  on  the  contrary  do  not  possess 
this  quality.  There  are  but  few  exceptions  to  the  rule.  Carbon 
is  one  of  them.  Becquerel  noted  that  carbon  phosphoresces  at 
at  a  certain  elevated  temperature  preceding  the  dark  red.  This 
phenomenon  may  be  easily  observed  in  bulbs  provided  with  a 
rather  large  carbon  electrode  (say,  a  sphere  of  six  millimetres  di- 
ameter). If  the  current  is  turned  on  after  a  few  seconds,  a  snow 
white  film  covers  the  electrode,  just  before  it  gets  dark  red. 
Similar  effects  are  noted  with  other  conducting  bodies,  but  many 
scientific  men  will  probably  not  attribute  them  to  true  phosphor- 
escence. Whether  true  incandescence  has  anything  to  do  with 
phosphorescence  excited  by  atomic  impact  or  mechanical  shocks 
still  remains  to  be  decided,  but  it  is  a  fact  that  all  conditions, 
wThich  tend  to  localize  and  increase  the  heating  effect  at  the  point 
of  impact,  are  almost  invariably  the  most  favorable  for  the  pro- 
duction of  phosphorescence.  So,  if  the  electrode  be  very  small, 
which  is  equivalent  to  saying  in  general,  that  the  electric  density 
is  great ;  if  the  potential  be  high,  arid  if  the  gas  be  highly  rare- 
fied, all  of  which  things  imply  high  speed  of  the  projected  atoms, 
or  matter,  and  consequently  violent  impacts — the  phosphores- 
cence is  very  intense.  If  a  bulb  provided  with  a  large  and  small 
electrode  be  attached  to  the  terminal  of  an  induction  coil,  the 
small  electrode  excites  phosphorescence  while  the  large  one  may 
not  do  so,  because  of  the  smaller  electric  density  and  hence 
smaller  speed  of  the  atoms.  A  bulb  provided  with  a  large  elec- 
trode may  be  grasped  with  the  hand  Avhile  the  electrode  is  con- 
nected to  the  terminal  of  the  coil  and  it  may  not  phosphoresce ; 
but  if  instead  of  grasping  the  bulb  with  the  hand,  the  same  be 
touched  with  a  pointed  wire,  the  phosphorescence  at  once  spreads 


11IG1I  VllI&Q UENCY  A ND  HIGH  POTENTIAL  CURRKltTS.      307 

through  the  bulb,  because  of  the  great  density  at  the  point  of 
contact.  .With  low  frequencies  it  seems  that  gases  of  great 
atomic  weight  excite  more  intense  phosphorescence  than  those 
of  smaller  weight,  as  for  instance,  hydrogen.  With  high  fre- 
quencies the  observations  are  not  sufficiently  reliable  to  draw  a 
conclusion.  Oxygen,  as  is  well-known,  produces  exceptionally 
strong  effects,  which  may  be  in  part  due  to  chemical  action.  A 
bulb  with  hydrogen  residue  seems  to  be  most  easily  excited. 
Electrodes  which  are  most  easily  deteriorated  produce  more 
intense  phosphorescence  in  bulbs,  but  the  condition  is  not  per- 
manent because  of  the  impairment  of  the  vacuum  and  the  deposi- 
tion of  the  electrode  matter  upon  the  phosphorescent  surfaces. 
Some  liquids,  as  oils,  for  instance,  produce  magnificent  effects  of 
phosphorescence  (or  fluorescence  ?),  but  they  last  only  a  few 
{seconds.  So  if  a  bulb  has  a  trace  of  oil  on  the  walls  and  the 
current  is  turned  on,  the  phosphorescence  only  persists  for  a  few 
moments  until  the  oil  is  carried  away.  Of  all  bodies  so  far  tried, 
sulphide  of  zinc  seems  to  be  the  most  susceptible  to  phosphores- 
cence. Some  samples,  obtained  through  the  kindness  of  Prof. 
Henry  in  Paris,  were  employed  in  many  of  these  bulbs.  One  of 
the  defects  of  this  sulphide  is,  that  it  loses  its  quality  of  emitting 
light  when  brought  to  a  temperature  which  is  by  no  means  high. 
It  can  therefore,  be  used  only  for  feeble  intensities.  An  obser- 
vation which  might  deserve  notice  is,  that  when  violently  bom- 
barded from  an  aluminum  electrode  it  assumes  a  black  color,  but 
singularly  enough,  it  returns  to  the  original  condition  when  it 
cools  down. 

The  most  important  fact  arrived  at  in  pursuing  investigations 
in  this  direction  is,  that  in  all  cases  it  is  necessary,  in  order  to  ex- 
cite phosphorescence  with  a  minimum  amount  of  energy,  to  ob- 
serve certain  conditions.  Namely,  there  is  always,  no  matter  what 
the  frequency  of  the  currents,  degree  of  exhaustion  and  character 
of  the  bodies  in  the  bulb,  a  certain  potential  (assuming  the  bulb 
excited  from  one  terminal)  or  potential  difference  (assuming  the 
bulb  to  be  excited  with  both  terminals)  which  produces  the  most 
economical  result.  If  the  potential  be  increased,  considerable 
energy  may  be  wasted  without  producing  any  more  light,  and  if 
it  be  diminished,  then  again  the  light  production  is  not  as  econom- 
ical. The  exact  condition  under  which  the  best  result  is  obtained 
seems  to  depend  on  many  things  of  a  different  nature,  and  it  is  to 
be  yet  investigated  by  other  experimenters,  but  it  will  certainly 


368  INVENTIONS  OF  NIKOLA  TESLA. 

have  to  be  observed  when  such  phosphorescent  bulbs  are  oper- 
ated, if  the  best  results  are  to  be  obtained. 

Coming  now  to  the  most  interesting  of  these  phenomena,  the 
incandescence  or  phosphorescence  of  gases,  at  low  pressures  or  at 
the  ordinary  pressure  of  the  atmosphere,  we  must  seek  the  ex- 
planation of  these  phenomena  in  the  same  primary  causes,  that  is, 
in  shocks  or  impacts  of  the  atoms.  Just  as  molecules  or  atoms 
beating  upon  a  solid  body  excite  phosphorescence  in  the  same  or 
render  it  incandescent,  so  when  colliding  among  themselves  they 
produce  similar  phenomena.  But  this  is  a  very  insufficient  ex- 
planation and  concerns  only  the  crude  mechanism.  Light  is  pro- 
duced by  vibrations  which  go  on  at  a  rate  almost  inconceivable. 
If  we  compute,  from  the  energy  contained  in  the  form  of  known 
radiations  in  a  definite  space  the  force  which  is  necessary  to  set 
up  such  rapid  vibrations,  we  find,  that  though  the  density  of  the 
ether  be  incomparably  smaller  than  that  of  any  body  we  know, 
even  hydrogen,  the  force  is  something  surpassing  comprehension. 
What  is  this  force,  which  in  mechanical  measure  may  amount  to 
thousands  of  tons  per  square  inch  ?  It  is  electrostatic  force  in  the 
light  of  modern  views.  It  is  impossible  to  conceive  how  a  body 
of  measurable  dimensions  could  be  charged  to  so  high  a  potential 
that  the  force  would  be  sufficient  to  produce  these  vibrations. 
Long  before  any  such  charge  could  be  imparted  to  the  body  it 
would  be  shattered  into  atoms.  The  sun  emits  light  and  heat,  and 
so  does  an  ordinary  flame  or  incandescent  filament,  but  in  neither 
of  these  can  the  force  be  accounted  for  if  it  be  assumed  that  it  is 
associated  with  the  body  as  a  whole.  Only  in  one  way  may  we 
account  for  it,  namely,  by  identifying  it  with  the  atom.  An 
atom  is  so  small,  that  if  it  be  charged  by  coming  in  contact  with 
an  electrified  body  and  the  charge  be  assumed  to  follow  the  same 
law  as  in  the  case  of  bodies  of  measurable  dimensions,  it  must 
retain  a  quantity  of  electricity  which  is  fully  capable  of  account- 
ing for  these  forces  and  tremendous  rates  of  vibration.  But  the 
atom  behaves  singularly  in  this  respect — it  always  takes  the  same 
"  charge." 

It  is  very  likely  that  resonant  vibration  plays  a  most  important 
part  in  all  manifestations  of  energy  in  nature.  Throughout  space 
all  matter  is  vibrating,  and  all  rates  of  vibration  are  represented, 
from  the  lowest  musical  note  to  the  highest  pitch  of  the  chemical 
rays,  hence  an  atom,  or  complex  of  atoms,  no  matter  what  its 
period,  must  find  a  vibration  with  which  it  is  in  resonance. 


HIGH  FREQUENCY  AND  HIGH  POTENTIAL  CURRENTS.      869 

"When  we  consider  the  enormous  rapidity  of  the  light  vibrations, 
we  realize  the  impossibility  of  producing  such  vibrations  directly 
with  any  apparatus  of  measurable  dimensions,  and  we  are  driven 
to  the  only  possible  means  of  attaining  the  object  of  setting  up 
waves  of  light  by  electrical  means  and  economically,  that  is,  to 
affect  the  molecules  or  atoms  of  a  gas,  to  cause  them  to  collide  and 
vibrate.  We  then  must  ask  ourselves — How  can  free  molecules 
or  atoms  .be  affected  ? 

It  is  a  fact  that  they  can  be  affected  by  electrostatic  force,  as  is 
apparent  in  many  of  these  experiments.  By  varying  the  electro- 
static force  we  can  agitate  the  atoms,  and  cause  them  to  collide 
accompanied  by  evolution  of  heat  and  light.  It  is  not  demonstrated 
beyond  doubt  that  we  can  affect  them  otherwise.  If  a  luminous 
discharge  is  produced  in  a  closed  exhausted  tube,  do  the  atoms 
arrange  themselves  in  obedience  to  any  other  but  to  electrostatic 
force  acting  in  straight  lines  from  atom  to  atom  ?  Only  recently 
I  investigated  the  mutual  action  between  two  circuits  with  extreme 
rates  of  vibration.  When  a  battery  of  a  few  jars  (c  c  c  c,  Fig. 
196)  is  discharged  through  a  primary  p  of  low  resistance  (the  con- 
nections being  as  illustrated  in  Figs.  183«,  183&andl83c),  and  the 
frequency  of  vibration  is  many  millions  there  are  great  differ- 
ences of  potential  between  points  on  the  primary  not  more  than 
a  few  inches  apart.  These  differences  may  be  10,000  volts  per 
inch,  if  not  more,  taking  the  maximum  value  of  the  E.  M.  F.  The 
secondary  s  is  therefore  acted  upon  by  electrostatic  induction, 
which  is  in  such  extreme  cases  of  much  greater  importance  than 
the  electro-dynamic.  To  such  sudden  impulses  the  primary  as 
well  as  the  secondary  are  poor  conductors,  and  therefore  great 
differences  of  potential  may  be  produced  by  electrostatic  induc- 
tion between  adjacent  points  on  the  secondary.  Then  sparks  may 
jump  between  the  wires  and  streamers  become  visible  in  the  dark 
if  the  light  of  the  discharge  through  the  spark  gap  c?  c?  be  carefully 
excluded.  If  now  we  substitute  a  closed  vacuum  tube  for  the 
metallic  secondary  s,  the  differences  of  potential  produced  in  the 
tube  by  electrostatic  induction  from  the  primary  are  fully  suffi- 
cient to  excite  portions  of  it ;  but  as  the  points  of  certain  differ- 
ences of  potential  on  the  primary  are  not  fixed,  but  are  generally 
constantly  changing  in  position,  a  luminous  band  is  produced  in 
the  tube,  apparently  not  touching  the  glass,  as  it  should,  if  the 
points  of  maximum  and  minimum  differences  of  potential  were 
fixed  on  the  primary.  I  do  not  exclude  the  possibility  of  such  a 


370  INVENTIONS  OF  NIKOLA  TESLA. 

tube  being  excited  only  by  electro-dynamic  induction,  for  very 
able  physicists  hold  this  view ;  but  in  my  opinion,  there  is  as  yet 
no  positive  proof  given  that  atoms  of  a  gas  in  a  closed  tube  may 
arrange  themselves  in  chains  under  the  action  of  an  electromotive 
impulse  produced  by  electro-dynamic  induction  in  the  tube.  I 
have  been  unable  so  far  to  produce  striae  in  a  tube,  however  long, 
and  at  whatever  degree  of  exhaustion,  that  is,  striae  at  right 
angles  to  the  supposed  direction  of  the  discharge  or  the  axis  of 
the  tube  ;  but  I  have  distinctly  observed  in  a  large  bulb,  in  which 
a  wide  luminous  band  was  produced  by  passing  a  discharge  of  a 
battery  through  a  wire  surrounding  the  bulb,  a  circle  of  feeble 
luminosity  between  two  luminous  bands,  one  of  which  was  more 
intense  than  the  other.  Furthermore,  with  my  present  experi- 
ence I  do  not  think  that  such  a  gas  discharge  in  a  closed  tube 
can  vibrate,  that  is,  vibrate  as  a  whole.  I  am  convinced  that  no 


V 


FIG.  196.  FIG.  197. 

discharge  through  a  gas  can  vibrate.  The  atoms  of  a  gas  behave 
very  curiously  in  respect  to  sudden  electric  impulses.  The 
gas  does  not  seem  to  possess  any  appreciable  inertia  to  such 
impulses,  for  it  is  a  fact,  that  the  higher  the  frequency  of 
the  impulses,  with  the  greater  freedom  does  the  discharge 
pass  through  the  gas.  If  the  gas  possesses  no  inertia  thqui 
it  cannot  vibrate,  for  some  inertia  is  necessary  for  the  free  vibra- 
tion. I  conclude  from  this  that  if  a  lightning  discharge  occurs 
between  two  clouds,  there  can  be  no  oscillation,  such  as  would 
be  expected,  considering  the  capacity  of  the  clouds.  But  if 
the  lightning  discharge  strike  the  earth,  there  is  always  vibra- 
tion— in  the  earth,  but  not  in  the  cloud.  In  a  gas  discharge  each 
atom  vibrates  at  its  own  rate,  but  there  is  no  vibration  of  the 
conducting  gaseous  mass  as  a  whole.  This  is  an  important 
consideration  in  the  great  problem  of  producing  light  economi- 


HIGH  FREQUENCY  AND  HIGH  POTENTIAL  CURRENTS.      371 

callj,  for  it  teaches  us  that  to  reach  this  result  we  must  use 
impulses  of  very  high  frequency  and  necessarily  also  of  high 
potential.  It  is  a  fact  that  oxygen  produces  a  more  intense 
light  in  a  tube.  Is  it  because  oxygen  atoms  possess  some  inertia 
and  the  vibration  does  not  die  out  instantly  ?  But  then  nitrogen 
should  be  as  good,  and  chlorine  and  vapors  of  many  other  bodies 
much  better  than  oxygen,  unless  the  magnetic  properties  of  the 
latter  enter  prominently  into  play.  Or,  is  the  process  in  the  tube 
of  an  electrolytic  nature  ?  Many  observations  certainly  speak  for 
it,  the  most  important  being  that  matter  is  always  carried  away 
from  the  electrodes  and  the  vacuum  in  a  bulb  cannot  be  perma- 
nently maintained.  If  such  process  takes  place  in  reality,  then 
again  must  we  take  refuge  in  high  frequencies,  for,  with  such, 
electrolytic  action  should  be  reduced  to  a  minimum,  if  not  ren_ 
dered  entirely  impossible.  It  is  an  undeniable  fact  that  with  very 
high  frequencies,  provided  the  impulses  be  of  harmonic  nature, 
like  those  obtained  from  an  alternator,  there  is  less  deteri- 
oration and  the  vacua  are  more  permanent.  With  disruptive  dis- 
charge coils  there  are  sudden  rises  of  potential  and  the  vacua  are 
more  quickly  impaired,  for  the  electrodes  are  deteriorated  in  a 
very  short  time.  It  was  observed  in  some  large  tubes,  which 
were  provided  with  heavy  carbon  blocks  B  Bl5  connected  to  plati- 
num wires  w  w^  (as  illustrated  in  Fig.  197),  and  which  were  em- 
ployed in  experiments  with  the  disruptive  discharge  instead  of  the 
ordinary  air  gap,  that  the  carbon  particles  under  the  action  of  the 
powerful  magnetic  field  in  which  the  tube  was  placed,  were  de- 
posited in  regular  fine  lines  in  the  middle  of  the  tube,  as  illus- 
trated. These  lines  were  attributed  to  the  deflection  or  distortion 
of  the  discharge  by  the  magnetic  field,  but  why  the  deposit 
occiirred  principally  where  the  field  was  most  intense  did  not 
appear  quite  clear.  A  fact  of  interest,  likewise  noted,  was 
that  the  presence  of  a  strong  magnetic  field  increases  the  deteri- 
oration of  the  electrodes,  probably  by  reason  of  the  rapid  inter- 
ruptions it  produces,  whereby  there  is  actually  a  higher  E.  M.  F. 
maintained  between  the  electrodes. 

Much  would  remain  to  be  said  about  the  luminous  effects  pro- 
duced in  gases  at  low  or  ordinary  pressures.  With  the  present 
experiences  before  us  we  cannot  say  that  the  essential  nature  of 
these  charming  phenomena  is  sufficiently  known.  But  investiga- 
tions in  this  direction  are  being  pushed  with  exceptional  ardor. 
Every  line  of  scientific  pursuit  has  its  fascinations,  but  electrical 


372 


INVENTIONS  OF  NIKOLA  TESLA. 


investigation  appears  to  possess  a  peculiar  attraction,  for  there  is 
no  experiment  or  observation  of  any  kind  in  the  domain  of  this 
wonderful  science  which  Avould  not  forcibly  appeal  to  us.  Yet 
to  me  it  seems,  that  of  all  the  many  marvelous  things  we  observe, 
a  vacuum  tube,  excited  by  an  electric  impulse  from  a  distant 
source,  bursting  forth  out  of  the  darkness  and  illuminating  the 
room  with  its  beautiful  light,  is  as  lovely  a  phenomenon  as  can 
greet  our  eyes.  More  interesting  still  it  appears  when,  reducing 
the  fundamental  discharges  across  the  gap  to  a  very  small  nuiu- 


FIG.  198. 

ber  and  waving  the  tube  about  we  produce  all  kinds  of  designs 
in  luminous  lines.  So  by  way  of  amusement  I  take  a  straight 
long  tube,  or  a  square  one,  or  a  square  attached  to  a  straight  tube, 
and  by  whirling  them  about  in  the  hand,  I  imitate  the  spokes  of 
a  wheel,  a  Gramme  winding,  a  drum  winding,  an  alternate  cur- 
rent motor  winding,  etc.  (Fig.  198).  Viewed  from  a  distance  the 
effect  is  weak  and  much  of  its  beauty  is  lost,  but  being  near  or 
holding  the  tube  in  the  hand,  one  cannot  resist  its  charm. 


HIGH  FREQUENCY  AND  1IIQR  POTENTIAL  CURRENTS.      373 

In  presenting  these  insignificant  results  I  have  not  attempted 
to  arrange  and  co-ordinate  them,  as  would  be  proper  in  a  strictly 
scientific  investigation,  in  which  every  succeeding  result  should 
be  a  logical  sequence  of  the  preceding,  so  that  it  might  be  guessed 
in  advance  by  the  careful  reader  or  attentive  listener.  I  have 
preferred  to  concentrate  my  energies  chiefly  upon  advancing 
novel  facts  or  ideas  which  might  serve  as  suggestions  to  others, 
and  this  may  serve  as  an  excuse  for  the  lack  of  harmony.  The 
explanations  of  the  phenomena  have  been  given  in  good  faith 
and  in  the  spirit  of  a  student  prepared  to  find  that  they  admit  of 
a  better  interpretation.  There  can  be  no  great  harm  in  a  student 
taking  an  erroneous  view,  but  when  great  minds  err,  the  world 
must  dearly  pay  for  their  mistakes. 


CHAPTEK   XXIX. 

TESLA   ALTERNATING    CURRENT    GENERATORS    FOR    HIGH    FRE- 
QUENCY, IN  DETAIL. 

It  lias  become  a  common  practice  to  operate  arc  lamps  by  alter- 
nating or  pulsating,  as  distinguished  from  continuous,  currents  ; 
but  an  objection  which  has  been  raised  to  such  systems  exists  in 
the  fact  that  the  arcs  emit  a  pronounced  sound,  varying  with  the 
rate  of  the  alternations  or  pulsations  of  current.  This  noise  is 
due  to  the  rapidly  alternating  heating  and  cooling,  and  conse- 
quent expansion  and  contraction,  of  the  gaseous  matter  forming- 
the  arc,  which  corresponds  with  the  periods  or  impulses  of  the 
current.  Another  disadvantageous  feature  is  found  in  the  diffi- 
culty of  maintaining  an  alternating  current  arc  in  consequence  of 
the  periodical  increase  in  resistance  corresponding  to  the  periodi- 
cal working  of  the  current.  This  feature  entails  a  further  dis- 
advantage, namely,  that  small  arcs  are  impracticable. 

Theoretical  considerations  have  led  Mr.  Tesla  to  the  belief 
that  these  disadvantageous  features  could  be  obviated  by  employ- 
ing currents  of  a  sufficiently  high  number  of  alternations,  and  his 
anticipations  have  been  confirmed  in  practice.  These  rapidly 
alternating  currents  render  it  possible  to  maintain  small  arcs 
which,  besides,  possess  the  advantages  of  silence  and  persistency. 
The  latter  quality  is  due  to  the  necessarily  rapid  alternation^  in 
consequence  of  which  the  arc  has  no  time  to  cool,  and  is  always 
maintained  at  a  high  temperature  and  low  resistance. 

At  the  outset  of  his  experiments  Mr.  Tesla  encountered  great 
difficulties  in  the  construction  of  high  frequency  machines.  A 
generator  of  this  kind  is  described  here,  which,  though  con- 
structed quite  some  time  ago,  is  well  worthy  of  a  detailed  de- 
scription. It  may  be  mentioned,  in  passing,  that  dynamos  of 
this  type  have  been  used  by  Mr.  Tesla  in  his  lighting  researches 
and  experiments  with  currents  of  high  potential  and  high  fre- 
quency, and  reference  to  them  will  be  found  in  his  lectures 
elsewhere  printed  in  this  volume.1 

1.  See  pages  153-4  5. 


HIGH  FREQUENCY  AND  HIGH  POTENTIAL  CURRENTS.     375 

In  the  aecompaning  engravings,  Figs.  199  and  200  show  the 
machine,  respectively,  in  side  elevation  and  vertical  cross-section  ; 
Figs.  201,  202  and  203  showing  enlarged  details  of  construction. 
As  will  be  seen,  A  is  an  annular  magnetic  frame,  the  interior  of 
which  is  provided  with  a  large  number  of  pole-pieces  D. 

Owing  to  the  very  large  number  and  small  size  of  the  poles 
and  the  spaces  between  them,  the  field  coils  are  applied  by  wind- 
ing an  insulated  conductor  F  zigzag  through  the  grooves,  as  shown 
in  Fig.  203,  carrying  the  wire  around  the  annulus  to  form  as 
many  layers  as  is  desired.  In  this  way  the  pole-pieces  D  will  be 
energized  with  alternately  opposite  polarity  around  the  entire 
ring. 

For  the  armature,  Mr.  Tesla  employs  a  spider  carrying  a  ring 


j,  turned  down,  except  at  its  edges,  to  form  a  trough-like  recep- 
tacle for  a  mass  of  fine  annealed  iron  wires  K,  which  are  wound 
in  the  groove  to  form  the  core  proper  for  the  armature-coils. 
Pins  L  are  set  in  the  sides  of  the  ring  j  and  the  coils  M  are  wound 
over  the  periphery  of  the  armature-structure  and  around  the  pins. 
The  coils  M  are  connected  together  in  series,  and  these  terminals 
N  carried  through  the  hollow  shaft  H  to  contact-rings  P  P,  from 
which  the  currents  are  taken  off  by  brushes  o. 

In  this  way  a  machine  with  a  very  large  number  of  poles  may 
be  constructed.  It  is  easy,  for  instance,  to  obtain  in  this  manner 
three  hundred  and  seventy-five  to  four  hundred  poles  in  a  machine 
that  may  be  safely  driven  at  a  speed  of  fifteen  hundred  or  six- 
teen hundred  revolutions  per  minute,  which  will  produce  ten 


376 


INVENTIONS  OF  NIKOLA  TESLA, 


thousand  or  eleven  thousand  alternations  of  current  per  second. 
Arc  lamps  K  R  are  shown  in  the  diagram  as  connected  up  in  series 
with  the  machine  in  Fig.  200.  If  such  a  current  be  applied  to 
running  arc  lamps,  the  sound  produced  by  or  in  the  arc  becomes 
practically  inaudible,  for,  by  increasing  the  rate  of  change  in  the 
current,  and  consequently  the  number  of  vibrations  per  unit  of 
time  of  the  gaseous  material  of  the  arc  up  to,  or  beyond,  ten 
thousand  or  eleven  thousand  per  second,  or  to  what  is  regarded 
as  the  limit  of  audition,  the  sound  due  to  such  vibrations  will  not 
be  audible.  The  exact  number  of  changes  or  undulations  neces- 
sary to  produce  this  result  will  vary  somewhat  according  to  the 
size  of  the  arc — that  is  to  say,  the  smaller  the  arc,  the  greater  the 


FIGS.  200,  201,  202  and  203. 

number  of  changes  that  will  be  required  to  render  it  inaudible 
within  certain  limits.  It  should  also  be  stated  that  the  arc  should 
not  exceed  a  certain  length. 

The  difficulties  encountered  in  the  construction  of  these 
machines  are  of  a  mechanical  as  well  as  an  electrical  nature. 
The  machines  may  be  designed  on  two  plans :  the  field  may  be 
formed  either  of  alternating  poles,  or  of  polar  projections  of  the 
same  polarity.  Up  to  about  15,000  alternations  per  second  in  an 
experimental  machine,  the  former  plan  may  be  followed,  but  a 
more  efficient  machine  is  obtained  on  the  second  plan. 

In  the  machine  above  described,  which  was  capable  of  running 
two  arcs  of  normal  candle  power,  the  field  was  composed  of  a 


HIGH  FREQUENCY  AND  HLOU  POTENTIAL  CURRENTS.      37? 

ring  of  wrought  iron  32  inches  outside  diameter,  and  about  1 
inch  thick.  The  inside  diameter  was  30  inches.  There  were  384 
polar  projections.  The  wire  was  wound  in  zigzag  form,  but  two 
wires  were  wound  so  as  to  completely  envelop  the  projections. 
The  distance  between  the  pro  jections  is  about  T3^  inch,  and  they 
are  a  little  over  j\  inch  thick.  The  field  magnet  was  made  rela- 
tively small  so  as  to  adapt  the  machine  for  a  constant  current. 
There  are  384  coils  connected  in  two  series.  It  was  found  im- 
practicable to  use  any  wire  much  thicker  than  No.  26  B.  and  S. 
gauge  on  account  of  the  local  effects.  In  such  a  machine  the 
clearance  should  be  as  small  as  possible;  for  this  reason  the 
machine  was  made  only  1  £  inch  wide,  so  that  the  binding  wires 
might  be  obviated.  The  armature  wires  must  be  wound  with 


FIG.  204. 

great  care,  as  they  are  apt  to  fly  off  in  consequence  of  the  great 
peripheral  speed.  In  various  experiments  this  machine  has  been 
run  as  high  as  3,000  revolutions  per  minute.  Owing  to  the  great 
speed  it  was  possible  to  obtain  as  high  as  10  amperes  out  of  the 
machine.  The  electromotive  force  was  regulated  by  means  of 
an  adjustable  condenser  within  very  wide  limits,  the  limits 
being  the  greater,  the  greater  the  speed.  This  machine  was 
frequently  used  to  run  Mr.  Tesla's  laboratory  lights. 

The  machine  above  described  was  only  one  of  many  such 
types  constructed.  It  serves  well  for  an  experimental  machine, 
but  if  still  higher  alternations  are  required  and  higher  efficiency 
is  necessary,  then  a  machine  on  a  plan  shown  in  Figs.  204  to 


378 


INVENTIONS  OF  NIKOLA  TE8LA. 


207,  is  preferable.  The  principal  advantage  of  this  type  of 
machine  is  that  there  is  not  much  magnetic  leakage,  and  that  a 
field  may  be  produced,  varying  greatly  in  intensity  in  places  not 
much  distant  from  each  other. 

In  these  engravings,  Figs.  204  and  205  illustrate  a  machine  in 
which  the  armature  conductor  and  field  coils  are  stationary,  while 
the  field  magnet  core  revolves.  Fig.  206  shows  a  machine 
embodying  the  same  plan  of  construction,  but  having  a  stationary 
field  magnet  and  rotary  armature. 

The  conductor  in  which  the  currents  are  induced  may  be 
arranged  in  various  ways ;  but  Mr.  Tesla  prefers  the  following 
method :  He  employs  an  annular  plate  of  copper  D,  and  by 


FIG.  205. 

means  of  a  saw  cuts  in  it  radial  slots  from  one  edge  nearly 
through  to  the  other,  beginning  alternately  from  opposite  edges. 
In  this  way  a  continuous  zigzag  conductor  is  formed.  When  the 
polar  projections  are  £  inch  wide,  the  width  of  the  conductor 
should  not,  under  any  circumstances,  be  more  than  ^  inch  wide ; 
even  then  the  eddy  effect  is  considerable. 

To  the  inner  edge  of  this  plate  are  secured  two  rings  of  non- 
magnetic metal  E,  which  are  insulated  from  the  copper  conductor, 
but  held  firmly  thereto  by  means  of  the  bolts  F.  Within  the 
rings  E  is  then  placed  an  annular  coil  G,  which  is  the  energizing 
coil  for  the  field  magnet.  The  conductor  D  and  the  parts  at- 
tached thereto  are  supported  by  means  of  the  cylindrical  shell  or 


HIGH  FREQ  UENCr  AND  HIGH  POTENTIA  L  CURRENTS.     379 

casting  A  A,  the  two  parts  of  which  are  brought  together  and 
clamped  to  the  outer  edge  of  the  conductor  D. 

The  core  for  the  field  magnet  is  built  up  of  two  circular  parts 
H  H,  formed  with  annular  grooves  i,  which,  when  the  two  parts 
are  brought  together,  form  a  space  for  the  reception  of  the  ener- 
gizing coil  G.  The  hubs  of  the  cores  are  trued  off,  so  as  to  fit 
closely  against  one  another,  while  the  outer  portions  or  flanges 
which  form  the  polar  faces  j  j,  are  reduced  somewhat  in  thick- 
ness to  make  room  for  the  conductor  D,  and  are  serrated  on  their 
faces.  The  number  of  serrations  in  the  polar  faces  is  arbitrary ; 


FIG.  206. 

but  there  must  exist  between  them  and  the  radial  portions  of 
the  conductor  D  certain  relation,  which  will  be  understood  by 
reference  to  Fig.  207  in  which  N  N  represent  the  projections  or 
points  on  one  face  of  the  core  of  the  field,  and  s  s  the  points  of 
the  other  face.  The  conductor  D  is  shown  in  this  figure  in  section 
a  a'  designating  the  radial  portions  of  the  conductor,  and  5  the 
insulating  divisions  between  them.  The  relative  width  of  the 
parts  a  a'  and  the  space  between  any  two  adjacent  points  N  N  or 
s  s  is  such  that  when  the  radial  portions  a  of  the  conductor  are 
passing  between  the  opposite  points  N  s  where  the  field  is  strong- 
est, the  intermediate  radial  portions  a'  are  passing  through  the 


380  INVENTIONS  OF  NIKOLA  TESLA. 

widest  spaces  midway  between  such  points  and  where  the  field  is 
weakest.  Since  the  core  on  one  side  is  of  opposite  polarity  to 
the  part  facing  it,  all  the  projections  of  one  polar  face  will  be  of 
opposite  polarity  to  those  of  the  other  face.  Hence,  although 
the  space  between  any  two  adjacent  points  on  the  same  face  may 
be  extremely  small,  there  will  be  no  leakage  of  the  magnetic 
lines  between  any  two  points  of  the  same  name,  but  the  lines  of 
force  will  pass  across  from  one  set  of  points  to  the  other.  The 
construction  followed  obviates  to  a  great  degree  the  distortion  of 
the  magnetic  lines  by  the  action  of  the  current  in  the  conductor 
D,  in  which  it  will  be  observed  the  current  is  flowing  at  any  given 
time  from  the  centre  toward  the  periphery  in  one  set  of  radial 
parts  a  and  in  the  opposite  direction  in  the  adjacent  parts  a'. 

In  order  to  connect  the  energizing  coil  G,  Fig.  204,  with  a  source 
of  continuous  current,  Mr.  Tesla  utilizes  two  adjacent  radial  por- 
tions of  the  conductor  D  for  connecting  the  terminals  of  the  coil 
G  with  two  binding  posts  M.  For  this  purpose  the  plate  D  is  cut 


vwwyy/ 

trm.mmmm  ..«.•'•> 


FIG.  207. 

entirely  through,  as  shown,  and  the  break  thus  made  is  bridged 
over  by  a  short  conductor  c.  The  plate  D  is  cut  through  to  form 
two  terminals  d,  which  are  connected  to  binding  posts  N.  The 
core  H  H,  when  rotated  by  the  driving  pulley,  generates  in  the  con- 
ductors D  an  alternating  current,  which  is  taken  off  from  the 
binding  posts  ST. 

When  it  is  desired  to  rotate  the  conductor  between  the  faces 
of  a  stationary  field  magnet,  the  construction  shown  in  Fig. 
206,  is  adopted.  The  conductor  D  in  this  case  is  or  may  be 
made  in  substantially  the  same  manner  as  above  described  by 
slotting  an  annular  conducting-plate  and  supporting  it  between 
two  heads  o,  held  together  by  bolts  o  and  fixed  to  the  driving-shaft 
K.  The  inner  edge  of  the  plate  or  conductor  D  is  preferably 
flanged  to  secure  a  firmer  union  between  it  and  the  heads  o.  It 
is  insulated  from  the  head.  The  field-magnet  in  this  case  con- 
sists of  two  annular  parts  H  H,  provided  with  annular  grooves  i 
for  the  reception  of  the  coils.  The  flanges  or  faces  surrounding 


HIGH  FREQUENCY  AND  HIGH  POTENTIAL  CURRENTS.     381 

the  annular  groove  are  brought  together,  while  the  inner  flanges 
are  serrated,  as  in  the  previous  case,  and  form  the  polar  faces. 
The  two  parts  H  H  are  formed  with  a  base  E,  upon  which  the 
machine  rests,  s  s  are  non-magnetic  bushings  secured  or  set  in 
the  central  opening  of  the  cores.  The  conductor  D  is  cut  entirely 
through  at  one  point  to  form  terminals,  from  which  insulated 
conductors  T  are  led  through  the  shaft  to  collecting-rings  v. 

In  one  type  of  machine  of  this  kind  constructed  by  Mr.  Tesla, 
the  field  had  480  polar  projections  on  each  side,  and  from  this 
machine  it  was  possible  to  obtain  30,000  alternations  per  second. 
As  the  polar  projections  must  necessarily  be  very  narrow,  very 
thin  wires  or  sheets  must  be  used  to  avoid  the  eddy  current 
effects.  Mr.  Tesla  has  thus  constructed  machines  with  a  station- 
ary armature  and  rotating  field,  in  which  case  also  the  field-coil 
was  supported  so  that  the  revolving  part  consisted  only  of  a 
wrought  iron  body  devoid  of  any  wire  and  also  machines  with  a 
rotating  armature  and  stationary  field.  The  machines  may  he 
either  drum  or  disc,  but  Mr.  Tesla's  experience  shows  the  latter 
to  be  preferable. 


In  the  course  of  a  very  interesting  article  contributed  to  the 
Electrical  World  in  February,  1891,  Mr.  Tesla  makes  some  sug- 
gestive remarks  on  these  high  frequency  machines  and  his  ex- 
periences with  them,  as  well  as  with  other  parts  of  the  high 
frequency  apparatus.  Part  of  it  is  quoted  here  and  is  as 
follows : — 

The  writer  will  incidentally  mention  that  any  one  who  at- 
tempts for  the  first  time  to  construct  such  a  machine  will  have  a 
tale  of  woe  to  tell.  He  will  first  start  out,  as  a  matter  of  course, 
by  making  an  armature  with  the  required  number  of  polar  pro- 
jections. He  will  then  get  the  satisfaction  of  having  produced 
an  apparatus  which  is  fit  to  accompany  a  thoroughly  Wagnerian 
opera.  It  may  besides  possess  the  virtue  of  converting  mechani- 
cal energy  into  heat  in  a  nearly  perfect  manner.  If  there  is  a 
reversal  in  the  polarity  of  the  projections,  he  will  get  heat  out  of 
the  machine ;  if  there  is  no  reversal,  the  heating  will  be  less,  but 
the  output  will  be  next  to  nothing.  He  will  then  abandon  the 
iron  in  the  armature,  and  he  will  get  from  the  Scylla  to  the 
Charybdis.  He  will  look  for  one  difficulty  and  will  find  another, 
but,  after  a  few  trials,  he  may  get  nearly  what  he  wanted. 


3*2  INVENTIONS  OF  NIKOLA  TK8LA. 

Among  the  many  experiments  winch  may  be  performed  with 
such  a  machine,  of  not  the  least  interest  are  those  performed 
with  a  high-tension  induction  coil.  The  character  of  the  dis- 
charge is  completely  changed.  The  arc  is  established  at  much 
greater  distances,  and  it  is  so  easily  affected  by  the  slightest  cur- 
rent of  air  that  it  often  wriggles  around  in  the  most  singular 
manner.  It  usually  emits  the  rhythmical  sound  peculiar  to  the 
alternate  current  arcs,  but  the  curious  point  is  that  the  sound 
may  be  heard  with  a  number  of  alternations  far  above  ten  thou- 
sand per  second,  which  by  many  is  considered  to  be  about  the 
limit  of  audition.  In  many  respects  the  coil  behaves  like  a  static 
machine.  Points  impair  considerably  the  sparking  interval,  elec- 
tricity escaping  from  them  freely,  and  from  a  wire  attached  to 
one  of  the  terminals  streams  of  light  issue,  as  though  it  were 
connected  to  a  pole  of  a  powerful  Toepler  machine.  All  these 
phenomena  are,  of  course,  mostly  due  to  the  enormous  differ- 
ences of  potential  obtained.  As  a  consequence  of  the  self-induc- 
tion of  the  coil  and  the  high  frequency,  the  current  is  minute 
while  there  is  a  corresponding  rise  of  pressure.  A  current  im- 
pulse of  some  strength  started  in  such  a  coil  should  persist  to 
flow  no  less  than  four  ten-thousandths  of  a  second.  As  this  time 
is  greater  than  half  the  period,  it  occurs  that  an  opposing  electro- 
motive force  begins  to  act  while  the  current  is  still  flowing.  As 
a  consequence,  the  pressure  rises  as  in  a  tube  filled  with  liquid 
and  vibrated  rapidly  around  its  axis.  The  current  is  so  small 
that,  in  the  opinion  and  involuntary  experience  of  the  writer,  the 
discharge  of  even  a  very  large  coil  cannot  produce  seriously  in- 
jurious effects,  whereas,  if  the  same  coil  were  operated  with  a 
current  of  lower  frequency,  though  the  electromotive  force  would 
be  much  smaller,  the  discharge  would  be  most  certainly  injuri- 
ous. This  result,  however,  is  due  in  part  to  the  high  frequency. 
The  writer's  experiences  tend  to  show  that  the  higher  the  fre- 
quency the  greater  the  amount  of  electrical  energy  which  may 
be  passed  through  the  body  without  serious  discomfort ;  whence 
it  seems  certain  that  human  tissues  act  as  condensers. 

One  is  not  quite  prepared  for  the  behavior  of  the  coil  when 
connected  to  a  Leyden  jar.  One,  of  course,  anticipates  that  since 
the  frequency  is  high  the  capacity  of  the  jar  should  be  small.  He 
therefore  takes  a  very  small  jar,  about  the  size  of  a  small  wine 
glass,  but  he  finds  that  even  with  this  jar  the  coil  is  practically 
short-circuited.  He  then  reduces  the  capacity  until  he  comes  to 


HIGH  FItEQ  UENCT  AND  HIG1I  POTENTIAL  CURliENTS,     383 

about  the  capacity  of  two  spheres,  say,  ten  centimetres  in  diam- 
eter and  two  to  four  centimetres  apart.  The  discharge  then  as- 
sumes the  form  of  a  serrated  band  exactly  like  a  succession  of 
sparks  viewed  in  a  rapidly  revolving  mirror ;  the  serrations,  of 
course,  corresponding  to  the  condenser  discharges.  In  this  case 
one  may  observe  a  queer  phenomenon.  The  discharge  starts  at 
the  nearest  points,  works  gradually  up,  breaks  somewhere  near 
the  top  of  the  spheres,  begins  again  at  the  bottom,  and  so  on. 
This  goes  on  so  fast  that  several  serrated  bands  are  seen  at  once. 
One  may  be  puzzled  for  a  few  minutes,  but  the  explanation  is 
simple  enough.  The  discharge  begins  at  the  nearest  points,  the  air 
is  heated  and  carries  the  arc  upward  until  it  breaks,  when  it  is  re- 
established at  the  nearest  points,  etc.  Since  the  current  passes 
easily  through  a  condenser  of  even  small  capacity,  it  will  be  found 
(juite  natural  that  connecting  only  one  terminal  to  a  body  of  the 
same  size,  no  matter  how  well  insulated,  impairs  considerably  the 
striking  distance  of  the  arc. 

Experiments  with  Greissler  tubes  are  of  special  interest.  An 
exhausted  tube,  devoid  of  electrodes  of  any  kind,  will  light  up  at 
some  distance  from  the  coil.  If  a  tube  from  a  vacuum  pump  is 
near  the  coil  the  whole  of  the  pump  is  brilliantly  lighted.  An 
incandescent  lamp  approached  to  the  coil  lights  up  and  gets  per- 
ceptibly hot.  If  a  lamp  have  the  terminals  connected  to  one  of 
the  binding  posts  of  the  coil  and  the  hand  is  approached  to  the 
bulb,  a  very  curious  and  rather  unpleasant  discharge  from  the 
glass  to  the  hand  takes  place,  and  the  filament  may  become  in- 
candescent. The  diSc"Iiarge  resembles  to  some  extent  the  stream 
issuing  from  the  plates  of  a  powerful  Toepler  machine,  but  is  of 
incomparably  greater  quantity.  The  lamp  in  this  case  acts  as  a 
condenser,  the  rarefied  gas  being  one  coating,  the  operator's  hand 
the  other.  By  taking  the  globe  of  a  lamp  in  the  hand,  and  by 
bringing  the  metallic  terminals  near  to  or  in  contact  with  a  con- 
ductor connected  to  the  coil,  the  carbon  is  brought  to  bright  in- 
candescence and  the  glass  is  rapidly  heated.  With  a  100- volt  10  c. 
p.  lanro  one  may  without  great  discomfort  stand  as  much  current 
as  will  bring  the  lamp  to  a  considerable  brilliancy ;  but  it  can  be 
held  in  the  hand  only  for  a  few  minutes,  as  the  glass  is  heated  in 
an  incredibly  short  time.  When  a  tube  is  lighted  by  bringing  it 
near  to  the  coil  it  may  be  made  to  go  out  by  interposing  a  metal 
plate  on  the  hand  between  the  coil  and  tube ;  but  if  the  metal 
plate  be  fastened  to  a  glass  rod  or  otherwise  insulated,  the  tube 


384  INVENTIONS  OF  NIKOLA  TE8LA. 

may  remain  lighted  if  the  plate  be  interposed,  or  may  even  in- 
crease in  luminosity.  The  effect  depends  on  the  position  of  the 
plate  and  tube  relatively  to  the  coil,  and  may  be  always  easily 
foretold  by  assuming  that  conduction  takes  place  from  one  ter- 
minal of  the  coil  to  the  other.  According  to  the  position  of  the 
plate,  it  may  either  divert  from  or  direct  the  current  to  the  tube. 
In  another  line  of  work  the  writer  has  in  frequent  experiments 
maintained  incandescent  lamps  of  50  or  100  volts  burning  at  any 
desired  candle  power  with  both  the  terminals  of  each  lamp  con- 
nected to  a  stout  copper  wire  of  no  more  than  a  few  feet  in 
length.  These  experiments  seem  interesting  enough,  but  they 
are  not  more  so  than  the  queer  experiment  of  Faraday,  which 
has  been  revived  and  made  much  of  by  recent  investigators,  and 
in  which  a  discharge  is  made  to  jump  between  two  points  of  a 
bent  copper  wire.  An  experiment  may  be  cited  here  which  may 
seem  equally  interesting.  If  a  Geissler  tube,  the  terminals  of 
which  are  joined  by  a  copper  wire,  be  approached  to  the  coil,  cer- 
tainly no  one  would  be  prepared  to  see  the  tube  light  up. 
Curiously  enough,  it  does  light  up,  and,  what  is  more,  the 
wire  does  not  seem  to  make  much  difference.  Now  one  is 
apt  to  think  in  the  first  moment  that  the  impedance  of  the 
wire  might  have  something  to  do  with  the  phenomenon.  But 
this  is  of  course  immediately  rejected,  as  for  this  an  enormous 
frequency  would  be  required.  This  result,  however,  seems 
puzzling  only  at  h'rst ;  for  upon  reflection  it  is  quite  clear  that 
the  wire  can  make  but  little  difference.  It  may  be  explained  in 
more  than  one  way,  but  it  agrees  perhaps  best  with  observation 
to  assume  that  conduction  takes  place  from  the  terminals  of  the 
coil  through  the  space.  On  this  assumption,  if  the  tube  with  the 
wire  be  held  in  any  position,  the  wire  can  divert  little  more  than 
the  current  which  passes  through  the  space  occupied  by  the  wire 
and  the  metallic  terminals  of  the  tube  ;  through  the  adjacent 
space  the  current  passes  practically  undisturbed.  For  this  reason, 
if  the  tube  be  held  in  any  position  at  right  angles  to  the  line 
joining  the  binding  posts  of  the  coil,  the  wire  makes  hardly  any 
difference,  but  in  a  position  more  or  less  parallel  with  that  line 
it  impairs  to  a  certain  extent  the  brilliancy  of  the  tube  and  its 
facility  to  light  up.  Numerous  other  phenomena  may  be  ex- 
plained on  the  same  assumption.  For  instance,  if  the  ends  of  the 
tube  be  provided  with  washers  of  sufficient  size  and  held  in  the 
line  joining  the  terminals  of  the  coil,  it  will  not  light  up,  and 
then  nearly  the  whole  of  the  current,  which  would  otherwise 


HIGH  FREQUENCY  AND  HIGH  POTENTIAL  CURRENTS.     385 

pass  uniformly  through  the  space  between  the  washers,  is  di- 
verted through  the  wire.  But  if  the  tube  be  inclined  sufficiently 
to  that  line,  it  will  light  up  in  spite  of  the  washers.  Also,  if  a 
metal  plate  be  fastened  upon  a  glass  rod  and  held  at  right  angles 
to  the  line  joining  the  binding  posts,  and  nearer  to  one  of  them, 
a  tube  held  more  or  less  parallel  with  the  line  will  light  up  in- 
stantly when  one  of  the  terminals  touches  the  plate,  and  will  go 
out  when  separated  from  the  plate.  The  greater  the  surface  of 
the  plate,  up  to  a  certain  limit,  the  easier  the  tube  will  light  up. 
When  a  tube  is  placed  at  right  angles  to  the  straight  line  joining 
the  binding  posts,  and  then  rotated,  its  luminosity  steadily  in- 
creases until  it  is  parallel  with  that  line.  The  writer  must  state, 
however,  that  he  does  nat  favor  the  idea  of  a  leakage  or  current 
through  the  space  any  more  than  as  a  suitable  explanation,  for  he 
is  convinced  that  all  these  experiments  could  not  be  performed  with 
a  static  machine  yielding  a  constant  difference  of  potential,  and 
that  condenser  action  is  largely  concerned  in  these  phenomena. 

It  is  well  to  take  certain  precautions  when  operating  a  Ruhm- 
korff  coil  with  very  rapidly  alternating  currents.  The  primary 
current  should  not  be  turned  on  too  long,  else  the  core  may  get 
so  hot  as  to  melt  the  gutta-percha  or  paraffin,  or  otherwise  injure 
the  insulation,  and  this  may  occur  in  a  surprisingly  short  time, 
considering  the  current's  strength.  The  primary  current  being 
turned  on,  the  tine  wire  terminals  may  be  joined  without  great 
risk,  the  impedance  being  so  great  that  it  is  difficult  to  force 
enough  current  through  the  fine  wire  so  as  to  injure  it,  and  in 
fact  the  coil  may  be  on  the  whole  much  safer  when  the  terminals 
of  the  fine  wire  are  connected  than  when  they  are  insulated ; 
but  special  care  should  be  taken  when  the  terminals  are  con- 
nected to  the  coatings  of  a  Leyden  jar,  for  with  anywhere  near 
the  critical  capacity,  which  just  counteracts  the  self-induction  at 
the  existing  frequency,  the  coil  might  meet  the  fate  of  St.  Poly- 
carpus.  If  an  expensive  vacuum  pump  is  lighted  up  by  being 
near  to  the  coil  or  touched  with  a  wire  connected  to  one  of  the 
terminals,  the  current  should  be  left  on  no  more  than  a  few 
moments,  else  the  glass  will  be  cracked  by  the  heating  of  the 
rarefied  gas  in  one  of  the  narrow  passages — in  the  writer's  own 
experience  quod  erat  demonstrandum.1 

1.  It  is  thought  necessary  to  remark  that,  although  the  induction  coil  may 
give  quite  a  good  result  when  operated  with  such  rapidly  alternating  currents, 
yet  its  construction,  quite  irrespective  of  the  iron  core,  makes  it  very  unfit  for 
such  high  frequencies,  and  to  obtain  the  best  results  the  construction  should  be 
greatly  modified. 


386  INVENTIONS  OF  NIKOLA  TESLA. 

There  are  a  good  many  other  points  of  interest  which  may  be 
observed  in  connection  with  such  a  machine.  Experiments  with 
the  telephone,  a  conductor  in  a  strong  field  or  with  a  condenser 
or  arc,  seem  to  afford  certain  proof  that  sounds  far  above  the 
usual  accepted  limit  of  hearing  would  be  perceived.  A  telephone 
will  emit  notes  of  twelve  to  thirteen  thousand  vibrations  per 
second ;  then  the  inability  of  the  core  to  follow  such  rapid  alter- 
nations begins  to  tell.  If,  however,  the  magnet  and  core  be 
replaced  by  a  condenser  and  the  terminals  connected  to  the  high- 
tension  secondary  of  a  transformer,  higher  notes  may  still  be 
heard.  If  the  current  be  sent  around  a  finely  laminated  core 
and  a  small  piece  of  thin  sheet  iron  be  held  gently  against  the 
•core,  a  sound  may  be  still  heard  with  thirteen  to  fourteen  thou- 
sand alternations  per  second,  provided  the  current  is  sufficiently 
strong.  A  small  coil,  however,  tightly  packed  between  the  poles 
of  a  powerful  magnet,  will  emit  a  sound  with  the  above  number 
of  alternations,  and  arcs  may  be  audible  with  a  still  higher  fre- 
quency. The  limit  of  audition  is  variously  estimated.  In  Sir 
William  Thomson's  writings  it  is  stated  somewhere  that  ten 
thousand  per  second,  or  nearly  so,  is  the  limit.  Other,  but  less 
reliable,  sources  give  it  as  high  as  twenty-four  thousand  per 
second.  The  above  experiments  have  convinced  the  writer  that 
notes  of  an  incomparably  higher  number  of  vibrations  per  second 
would  be  perceived  provided  they  could  be  produced  with  suffi- 
cient power.  There  is  no  reason  why  it  should  not  be  so.  The 
condensations  and  rarefactions  of  the  air  would  necessarily  set 
the  diaphragm  in  a  corresponding  vibration  and  some  sensation 
would  be  produced,  whatever — within  certain  limits — the  velocity 
of  transmission  to  their  nerve  centres,  though  it  is  probable  that 
for  want  of  exercise  the  ear  would  not  be  able  to  distinguish  any 
such  high  note.  With  the  eye  it  is  different ;  if  the  sense  of 
vision  is  based  upon  some  resonance  effect,  as  many  believe,  no 
amount  of  increase  in  the  intensity  of  the  ethereal  vibration 
could  extend  our  range  of  vision  on  either  side  of  the  visible 
spectrum. 

The  limit  of  audition  of  an  arc  depends  on  its  size.  The 
greater  the  surface  by  a  given  heating  effect  in  the  arc,  the  higher 
the  limit  of  audition.  The  highest  notes  are  emitted  by  the 
high-tension  discharges  of  an  induction  coil  in  which  the  arc  is, 
so  to  speak,  all  surface.  If  R  be  the  resistance  of  an  arc,  and  C 
the  current,  and  the  linear  dimensions  be  n  times  increased,  then 


HIGH  FREQUENCY  AND  HIGH  POTENTIAL  CURRENTS.     387 

the  resistance  is  — ,  and  with  the  same  current  density  the  cur- 
rent would  be  v?C;  hence  the  heating  effect  is  n*  times  greater, 
while  the  surface  is  only  n*  times  as  great.  For  this  reason  very 
large  arcs  would  not  emit  any  rhythmical  sound  even  with  a  very 
low  frequency.  It  must  be  observed,  however,  that  the  sound 
emitted  depends  to  some  extent  also  on  the  composition  of  the 
carbon.  If  the  carbon  contain  highly  refractory  material,  this, 
when  heated,  tends  to  maintain  the  temperature  of  the  arc  uni- 
form and  the  sound  is  lessened ;  for  this  reason  it  would  seem 
that  an  alternating  arc  requires  such  carbons. 

With  currents  of  such  high  frequencies  it  is  possible  to  obtain 
noiseless  arcs,  but  the  regulation  of  the  lamp  is  rendered  ex- 
tremely difficult  on  account  of  the  excessively  small  attractions 
or  repulsions  between  conductors  conveying  these  currents. 

An  interesting  feature  of  the  arc  produced  by  these  rapidly 
alternating  currents  is  its  persistency.  There  are  two  causes  for 
it,  one  of  which  is  always  present,  the  other  sometimes  only. 
One'  is  due  to  the  character  of  the  current  and  the  other  to  a 
property  of  the  machine.  The  first  cause  is  the  more  important 
one,  and  is  due  directly  to  the  rapidity  of  the  alternations. 
When  an  arc  is  formed  by  a  periodically  undulating  current, 
there  is  a  corresponding  undulation  in  the  temperature  of  the 
gaseous  column,  and,  therefore,  a  corresponding  undulation  in 
the  resistance  of  the  arc.  But  the  resistance  of  the  arc  varies 
enormously  with  the  temperature  of  the  gaseous  column,  being 
practically  infinite  when  the  gas  between  the  electrodes  is  cold. 
The  persistence  of  the  arc,  therefore,  depends  on  the  inability  of 
the  column  to  cool.  It  is  for  this  reason  impossible  to  maintain 
an  arc  with  the  current  alternating  only  a  few  times  a  second. 
On  the  other  hand,  with  a  practically  continuous  current,  the  arc 
is  easily  maintained,  the  column  being  constantly  kept  at  a  high 
temperature  and  low  resistance.  The  higher  the  frequency  the 
smaller  the  time  interval  during  which  the  arc  may  cool  and  in- 
crease considerably  in  resistance.  With  a  frequency  of  10,000 
per  second  or  more  in  an  arc  of  equal  size  excessively  small  varia- 
tions of  temperature  are  superimposed  upon  a  steady  temperature, 
like  ripples  011  the  surface  of  a  deep  sea.  The  heating  effect  is 
practically  continuous  and  the  arc  behaves  like  one  produced  by 
a  continuous  current,  with  the  exception,  however,  that  it  may 
not  be  quite  as  easily  started,  and  that  the  electrodes  are  equally 


888  INVENTIONS  OF  NIKOLA  TESLA. 

consumed ;  though  the  writer  has  observed  some  irregularities  in 
this  respect. 

The  second  cause  alluded  to,  which  possibly  may  not  be  pre- 
sent, is  due  to  the  tendency  of  a  machine  of  such  high  frequency 
to  maintain  a  practically  constant  current.  When  the  arc  is 
lengthened,  the  electromotive  force  rises  in  proportion  and  the 
arc  appears  to  be  more  persistent. 

Such  a  machine  is  eminently  adapted  to  maintain  a  constant 
current,  but  it  is  very  unfit  for  a  constant  potential.  As  a  matter 
of  fact,  in  certain  types  of  such  machines  a  nearly  constant  cur- 
rent is  an  almost  unavoidable  result.  As  the  number  of  poles  or 
polar  projections  is  greatly  increased,  the  clearance  becomes  of 
great  importance.  One  has  really  to  do  with  a  great  number  of 
very  small  machines.  Then  there  is  the  impedance  in  the  arma- 
ture, enormously  augmented  by  the  high  frequency.  Then, 
again,  the  magnetic  leakage  is  facilitated.  If  there  are  three  or 
four  hundred  alternate  poles,  the  leakage  is  so  great  that  it  is 
virtually  the  same  as  connecting,  in  a  two-pole  machine,  the  poles 
by  a  piece  of  iron.  This  disadvantage,  it  is  true,  may  be  obviated 
more  or  less  by  using  a  field  throughout  of  the  same  polarity, 
but  then  one  encounters  difficulties  of  a  different  nature.  All 
these  things  tend  to  maintain  a  constant  current  in  the  armature 
circuit. 

In  this  connection  it  is  interesting  to  notice  that  even  to-day 
engineers  are  astonished  at  the  performance  of  a  constant  current 
machine,  just  as,  some  years  ago,  they  used  to  consider  it  an  ex- 
traordinary performance  if  a  machine  was  capable  of  maintaining 
a  constant  potential  difference  between  the  terminals.  Yet  one 
result  is  just  as  easily  secured  as  the  other.  It  must  only  be 
remembered  that  in  an  inductive  apparatus  of  any  kind,  if  con- 
stant potential  is  required,  the  inductive  relation  between  the 
primary  or  exciting  and  secondary  or  armature  circuit  must  be 
the  closest  possible ;  whereas,  in  an  apparatus  for  constant  cur- 
rent just  the  opposite  is  required.  Furthermore,  the  opposition 
to  the  current's  flow  in  the  induced  circuit  must  be  as  small  as 
possible  in  the  former  and  as  great  as  possible  in  the  latter  case. 
But  opposition  to  a  current's  flow  may  be  caused  in  more  than 
one  way.  It  may  be  caused  by  ohmic  resistance  or  self-induc- 
tion. One  may  make  the  induced  circuit  of  a  dynamo  machine 
or  transformer  of  such  high  resistance  that  when  operating  de- 
vices of  considerably  smaller  resistance  within  very  wide  limits  a 


HIGH  FREQ  UENCY  AND  HIGH  POTENTIAL  CURRENTS.      389 

nearly  constant  current  is  maintained.  But  such  high  resistance 
involves  a  great  loss  in  power,  hence  it  is  not  practicable.  Not 
so  self-induction.  Self-induction  does  not  necessarily  mean  loss 
of  power.  The  moral  is,  use  self-induction  instead  of  resistance. 
There  is,  however,  a  circumstance  which  favors  the  adoption  of 
this  plan,  and  this  is,  that  a  very  high  self-induction  may  be 
obtained  cheaply  by  surrounding  a  comparatively  small  length 
of  wire  more  or  less  completely  with  iron,  and,  furthermore,  the 
effect  may  be  exalted  at  will  by  causing  a  rapid  undulation  of  the 
current.  To  sum  up,  the  requirements  for  constant  current 
are:  Weak  magnetic  connection  between  the  induced  and 
inducing  circuits,  greatest  possible  self-induction  with  the 
least  resistance,  greatest  practicable  rate  of  change  of  the 
current.  Constant  potential,  on  the  other  hand,  requires  :  Clos- 
est magnetic  connection  between  the  circuits,  steady  induced 
current,  and,  if  possible,  no  reaction.  If  the  latter  conditions 
could  be  .fully  satisfied  in  a  constant  potential  machine,  its  output 
would  surpass  many  times  that  of  a  machine  primarily  designed 
to  give  constant  current.  Unfortunately,  the  type  of  machine 
in  which  these  conditions  may  be  satisfied  is  of  little  practical 
value,  owing  to  the  small  electromotive  force  obtainable  and  the 
difficulties  iii  taking  off  the  current. 

With  their  keen  inventor's  instinct,  the  now  successful  arc- 
light  men  have  early  recognized  the  desiderata  of  a  constant 
current  machine.  Their  arc  light  machines  have  weak  fields, 
large  armatures,  with  a  great  length  of  copper  wire  and  few 
commutator  segments  to  produce  great  variations  in  the  current's 
strength  and  to  bring  self-induction  into  play.  Such  machines 
may  maintain  within  considerable  limits  of  variation  in  the  re- 
sistance of  the  circuit  a  practically  constant  current.  Their  out- 
put is  of  course  correspondingly  diminished,  and,  perhaps  with 
the  object  in  view  not  to  cut  down  the  output  too  much,  a  sim- 
ple device  compensating  exceptional  variations  is  employed. 
The  undulation  of  the  current  is  almost  essential  to  the  commer- 
cial success  of  an  arc-light  system.  It  introduces  in  the  circuit  a 
steadying  element  taking  the  place  of  a  large  ohmic  resistance, 
without  involving  a  great  loss  in  power,  and,  what  is  more  im- 
portant, it  allows  the  use  of  simple  clutch  lamps,  which  with  a 
current  of  a  certain  number  of  impulses  per  second,  best  suitable 
for  each  particular  lamp,  will,  if  properly  attended  to,  regulate 
even  better  than  the  finest  clock-work  lamps.  This  discovery 
has  been  made  by  the  writer — several  years  too  late. 


:-MK)  INVENTIONS  OF  NIKOLA  TESLA. 

It  has  been  asserted  by  competent  English  electricians  that  in  a 
constant-current  machine  or  transformer  the  regulation  is  effected 
by  varying  the  phase  of  the  secondary  current.  That  this  view 
is  erroneous  may  be  easily  proved  by  using,  instead  of  lamps,  de- 
vices each  possessing  self-induction  and  capacity  or  self-induction 
and  resistance — that  is,  retarding  and  accelerating  components — 
in  such  proportions  as  to  not  affect  materially  the  phase  of  the 
secondary  current.  Any  number  of  such  devices  may  be  inserted 
or  cut  out,  still  it  will  be  found  that  the  regulation  occurs,  a  con- 
stant current  being  maintained,  while  the  electromotive  force  is 
varied  with  the  number  of  the  devices.  The  change  of  phase  of 
the  secondary  current  is  simply  a  result  following  from  the 
changes  in  resistance,  and,  though  secondary  reaction  is  always 
of  more  or  less  importance,  yet  the  real  cause  of  the  regulation 
lies  in  the  existence  of  the  conditions  above  enumerated.  It 
should  be  stated,  however,  that  in  the  case  of  a  machine  the  above 
remarks  are  to  be  restricted  to  the  cases  in  which  the  machine  is 
independently  excited.  If  the  excitation  be  effected  by  commu- 
tating  the  armature  current,  then  the  iixed  position  of  the  brushes 
makes  any  shifting  of  the  neutral  line  of  the  utmost  importance, 
and  it  may  not  be  thought  immodest  of  the  writer  to  mention 
that,  as  far  as  records  go,  he  seems  to  have  been  the  first  who  has 
successfully  regulated  machines  by  providing  a  bridge  connection 
between  a  point  of  the  external  circuit  and  the  commutator  by 
means  of  a  third  brush.  The  armature  and  field  being  properly 
proportioned  and  the  brushes  placed  in  th  eir  determined  posi- 
tions, a  constant  current  or  constant  potential  resulted  from  the 
shifting  of  the  diameter  of  commutation  by  the  varying  loads. 

In  connection  with  machines  of  such  high  frequencies,  the 
condenser  affords  an  especially  interesting  study.  It  is  easy  to 
raise  the  electromotive  force  of  such  a  machine  to  four  or  five 
times  the  value  by  simply  connecting  the  condenser  to  the  cir- 
cuit, and  the  writer  has  continually  used  the  condenser  for  the 
the  purposes  of  regulation,  as  suggested  by  Blakesley  in  his  book 
on  alternate  currents,  in  which  he  has  treated  the  most  frequently 
occurring  condenser  problems  with  exquisite  simplicity  and  clear- 
ness. The  high  frequency  allowrs  the  use  of  small  capacities  and 
renders  investigation  easy.  But,  although  in  most  of  the  experi- 
ments the  result  may  be  foretold,  some  phenomena  observed  seem 
at  first  curious.  One  experiment  performed  three  or  four  months 
ago  with  such  a  machine  and  a  condenser  may  serve  as  an  il- 


HI&H  FREQUENCY  AND  HIGH  POTENTIAL  CURRENTS.      391 

lustration.  A  machine  was  used  giving  about  20,000  alternations 
per  second.  Two  bare  wires  about  twenty  feet  long  and  two 
millimetres  in  diameter,  in  close  proximity  to  each  other,  were 
connected  to  the  terminals  of  the  machine  at  the  one  end,  and 
to  a  condenser  at  the  other.  A  small  transformer  without  an 
iron  core,  of  course,  was  used  to  bring  the  reading  within  range 
of  a  Cardew  voltmeter  by  connecting  the  voltmeter  to  the 
secondary.  On  the  terminals  of  the  condenser  the  electromotive 
force  was  about  120  volts,  and  from  there  inch  by  inch  it  gradu- 
ally fell  until  at  the  terminals  of  the  machine  it  was  about  65 
volts.  It  was  virtually  as  though  the  condenser  were  a  gene- 
rator, and  the  line  and  armature  circuit  simply  a  resistance  con- 
nected to  it.  The  writer  looked  for  a  case  of  resonance,  but  he 
was  unable  to  augment  the  effect  by  varying  the  capacity  very 
carefully  and  gradually  or  by  changing  the  speed  of  the  ma- 
chine. A  case  of  pure  resonance  he  was  unable  to  obtain. 
When  a  condenser  was  connected  to  the  terminals  of  the  ma- 
chine— the  self-induction  of  the  armature  being  first  determined 
in  the  maximum  and  minimum  position  and  the  mean  value  taken 
— the  capacity  which  gave  the  highest  electromotive  force  corre- 
sponded most  nearly  to  that  which  just  counteracted  the  self-in- 
duction with  the  existing  frequency.  If  the  capacity  was  in- 
creased or  diminished,  the  electromotive  force  fell  as  expected. 

With  frequencies  as  high  as  the  above  mentioned,  the  con- 
denser effects  are  of  enormous  importance.  The  condenser 
becomes  a  highly  efficient  apparatus  capable  of  transferring 
considerable  energy. 


In  an  appendix  to  this  book  will  be  found  a  description  of  the 
Tesla  oscillator,  which  its  inventor  believes  will  among  other  great 
advantages  give  him  the  necessary  high  frequency  conditions, 
while  relieving  him  of  the  inconveniences  that  attach  to  genera- 
tors of  the  type  described  at  the  beginning  of  this  chapter. 


CHAPTEK  XXX. 

ALTERNATE  CURRENT  ELECTROSTATIC  INDUCTION  APPARATUS.* 

ABOUT  a  year  and  a  half  ago  while  engaged  in  the  study  of 
alternate  currents  of  short  period,  it  occurred  to  me  that  such 
currents  could  be  obtained  by  rotating  charged  surfaces  in  close 
proximity  to  conductors.  Accordingly  I  devised  various  forms 


FIG.  208. 

of  experimental  apparatus  of  which  two  are  illustrated  in  the 
accompanying  engravings. 

In  the  apparatus  shown  in  Fig.  208,  A  is  a  ring  of  dry  shel- 
lacked hard  wood  provided  on  its  inside  with  two  sets  of  tin-foil 
coatings,  a  and  J,  all  the  a  coatings  and  all  the  I  coatings  being 
connected  together,  respectively,  but  independent  from  each 
other.  These  two  sets  of  coatings  are  connected  to  two  termi- 

1.     Article  by  Mr.  Tesla  in  The  Electrical  Engineer,  N.  Y.,  May  6,  1891. 


HIGH  FREQUENCY  AND  HIGH  POTENTIAL  CURRENTS.     393 

nals,  T.  For  the  sake  of  clearness  only  a  few  coatings  are  shown. 
Inside  of  the  ring  A,  and  in  close  proximity  to  it  there  is  arranged 
to  rotate  a  cylinder  B,  likewise  of  dry,  shellacked  hard  wood,  and 
provided  with  two  similar  sets  of  coatings,  a1  and  J1,  all  the  coat- 
ings a}  being  connected  to  one  ring  and  all  the  others,  Sl,  to 
another  marked  -f-  and  — .  These  two  sets,  a1  and  J1  are  charged 
to  a  high  potential  by  a  Holtz  or  Wimshurst  machine,  and  may 
be  connected  to  a  jar  of  some  capacity.  The  inside  of  ring  A  is 
coated  with  mica  in  order  to  increase  the  induction  and  also  to 
allow  higher  potentials  to  be  used. 

When  the  cylinder  B  with  the  charged  coatings  is  rotated,  a 


FIG.  20«J. 


circuit  connected  to  the  terminals  T  is  traversed  by  alternating 
currents.  Another  form  of  apparatus  is  illustrated  in  Fig.  209. 
In  this  apparatus  the  two  sets  of  tin-foil  coatings  are  glued  on  a 
plate  of  ebonite,  and  a  similar  plate  which  is  rotated,  and  the 
coatings  of  which  are  charged  as  in  Fig.  208,  is  provided. 

The  output  of  such  an  apparatus  is  very  small,  but  some  of 
the  effects  peculiar  to  alternating  currents  of  short  periods  may 
l>e  observed.  The  effects,  however,  cannot  be  compared  with 
those  obtainable  with  an  induction  coil  which  is  operated  by  an 
alternate  current  machine  of  high  frequency,  some  of  which 
were  described  by  me  a  short  while  ago. 


CHAPTER  XXXI. 

"  MASSAGE  "   WITH  CURRENTS  OF  HIGH  FREQUENCY.1 

I  TRUST  that  the  present  brief  communication  will  not  be  inter- 
preted as  an  effort  on  my  part  to  put  myself  on  record  as  a 
"patent  medicine"  man,  for  a  serious  worker  cannot  despise 
anything  more  than  the  misuse  and  abuse  of  electricity  which  we 
have  frequent  occasion  to  witness.  My  remarks  are  elicited  by 
the  lively  interest  which  prominent  medical  practitioners  evince 
at  every  real  advance  in  electrical  investigation.  The  progress 
in  recent  years  has  been  so  great  that  every  electrician  and  elec- 
trical engineer  is  confident  that  electricity  will  become  the  means 
of  accomplishing  many  things  that  have  been  heretofore,  with 
our  existing  knowledge,  deemed  impossible.  ]Sro  wonder 'then 
that  progressive  physicians  also  should  expect  to  find  in  it  a 
powerful  tool  and  help  in  new  curative  processes.  Since  I  had 
the  honor  to  bring  before  the  American  Institute  of  Electrical 
Engineers  some  results  in  utilizing  alternating  currents  of  high 
tension,  I  have  received  many  letters  from  noted  physicians  in- 
quiring as  to  the  physical  effects  of  such  currents  of  high  fre- 
quency. It  may  be  remembered  that  I  then  demonstrated  that 
a  body  perfectly  well  insulated  in  air  can  be  heated  by  simply 
connecting  it  with  a  source  of  rapidly  alternating  high  potential. 
The  heating  in  this  case  is  due  in  all  probability  to  the  bombard- 
ment of  the  body  by  air,  or  possibly  by  some  other  medium, 
which  is  molecular  or  atomic  in  construction,  and  the  presence 
of  which  has  so  far  escaped  our  analysis — for  according  to  my 
ideas,  the  true  ether  radiation  with  such  frequencies  as  even  a 
few  millions  per  second  must  be  very  small.  This  body  may  be 
a  good  conductor  or  it  may  be  a  very  poor  conductor  of  elec- 
tricity with  little  change  in  the  result.  The  human  body  is,  in 
such  a  case,  a  fine  conductor,  and  if  a  person  insulated  in  a  room, 
or  no  matter  where,  is  brought  into  contact  with  such  a  source  of 

1.  Article  by  Mr.  Tesla  in  Tlie  EUctrical  Engineer  of  Deo.  23d,  1891. 


HIGH  FREQUENCY  AND  HIGH  POTENTIAL  CURRENTS.     395 

rapidly  alternating  high  potential,  the  skin  is  heated  by  bom- 
bardment. It  is  a  mere  question  of  the  dimensions  and  character 
of  the  apparatus  to  produce  any  degree  of  heating  desired. 

It  has  occurred  to  me  whether,  with  such  apparatus  properly 
prepared,  it  would  not  be  possible  for  a  skilled  physician  to  find 
in  it  a  means  for  the  effective  treatment  of  various  types  of  dis- 
ease. The  heating  will,  of  course,  be  superficial,  that  is,  on  the 
skin,  and  would  result,  whether  the  person  operated  on  were  in 
bed  or  walking  around  a  room,  whether  dressed  in  thick  clothes  or 
whether  reduced  to  nakedness.  In  fact,  to  put  it  broadly,  it  is 
conceivable  that  a  person  entirely  nude  at  the  North  Pole  might 
keep  himself  comfortably  warm  in  this  manner. 

Without  vouching  for  all  the  results,  which  must,  of  course,  be 
determined  by  experience  and  observation,  I  can  at  least  warrant 
the  fact  that  heating  would  occur  by  the  use  of  this  method  of 
subjecting  the  human  body  to  bombardment  by  alternating  cur- 
rents of  high  potential  and  frequency  such  as  I  have  long  worked 
with.  It  is  only  reasonable  to  expect  that  some  of  the  novel  ef- 
fects will  be  wholly  different  from  those  obtainable  with  the  old 
familiar  therapeutic  methods  generally  used.  Whether  they 
would  all  be  beneficial  or  not  remains  to  be  proved. 


CHAPTEE   XXXII. 

ELECTRIC  DISCHARGE  IN  VACUUM  TUBES.* 

IN  The  Electrical  Engineer  of  June  10  I  have  noted  the  de- 
scription of  some  experiments  of  Prof.  J.  J.  Thomson,  on  the 
"  Electric  Discharge  in  Vacuum  Tubes,"  and  in  your  issue  of  June 
24  Prof.  Elihu  Thomson  describes  an  experiment  of  the  same 
kind.  The  fundamental  idea  in  these  experiments  is  to  set  up 
an  electromotive  force  in  a  vacuum  tube — preferably  devoid  of 
any  electrodes — by  means  of  electro-magnetic  induction,  and  to 
excite  the  tube  in  this  manner. 

As  I  view  the  subject  I  should,  think  that  to  any  experimenter 
who  had  carefully  studied  the  problem  confronting  us  and  who 
attempted  to  find  a  solution  of  it,  this  idea  must  present  itself  as 
naturally  as,  for  instance,  the  idea  of  replacing  the  tinfoil  coat- 
ings of  a  Leyden  jar  by  rarefied  gas  and  exciting  luminosity  in 
the  condenser  thus  obtained  by  repeatedly  charging  and  discharg- 
ing it.  The  idea  being  obvious,  whatever  merit  there  is  in  this 
line  of  investigation  must  depend  upon  the  completeness  of  the 
study  of  the  subject  and  the  correctness  of  the  observations.  The 
following  lines  are  not  penned  with  any  desire  on  my  part  to  put 
myself  on  record  as  one  who  has  performed  similar  experiments, 
but  with  a  desire  to  assist  other  experimenters  by  pointing  out 
certain  peculiarities  of  the  phenomena  observed,  which,  to  all  ap- 
pearances, have  not  been  noted  by  Prof.  J.  J.  Thomson,  who, 
however,  seems  to  have  gone  about  systematically  in  his  investi- 
gations, and  who  has  been  the  first  to  make  his  results  known. 
These  peculiarities  noted  by  me  would  seem  to  be  at  variance 
with  the  views  of  Prof.  J.  J.  Thomson,  and  present  the  pheno- 
mena in  a  different  light. 

My  investigations  in  this  line  occupied  me  principally  during 
the  winter  and  spring  of  the  past  year.  During  this  time  many  dif- 
ferent experiments  were  performed,  and  in  my  exchanges  of  ideas 

1.     Article  by  Mr.  Tesla  in  The  Electrical  Engineer.  N.  Y.,  July  1,  1891. 


HIGH  FREQUENCY  AND  HIGH  POTENTIAL  CURRENTS.     397 

on  this  subject  with  Mr.  Alfred  S.  Brown,  of  the  "Western  Union 
Telegraph  Company,  various  different  dispositions  were  suggested 
which  were  carried  out  by  me  in  practice.  Fig.  210  may  serve 
as  an  example  of  one  of  the  many  forms  of  apparatus  used.  This 
consisted  of  a  large  glass  tube  sealed  at  one  end  and  projecting 
into  an  ordinary  incandescent  lamp  bulb.  The  primary,  usually 
consisting  of  a  few  turns  of  thick,  well-insulated  copper  sheet  was 
inserted  within  the  tube,  the  inside  space  of  the  bulb  furnishing 
the  secondary.  This  form  of  apparatus  was  arrived  at  after  some 
experimenting,  and  was  used  principally  with  the  view  of  en- 
abling me  to  place  a  polished  reflecting  surface  on  the  inside  of 
the  tube,  and  for  this  purpose  the  last  turn  of  the  primary  was 
covered  with  a  thin  silver  sheet.  In  all  forms  of  apparatus  used 


FIG.  210. 

there  was  no  special  difficulty  in  exciting  a  luminous  circle  or 
cylinder  in  proximity  to  the  primary. 

As  to  the  number  of  turns,  I  cannot  quite  understand  why 
Prof.  J.  J.  Thomson  should  think  that  a  few  turns  were  "quite 
sufficient,"  but  lest  I  should  impute  to  him  an  opinion  he  may 
not  have,  I  will  add  that  I  have  gained  this  impression  from  the 
reading  of  the  published  abstracts  of  his  lecture.  Clearly,  the 
number  of  turns  which  gives  the  best  result  in  any  case,  is  de- 
pendent on  the  dimensions  of  the  apparatus,  and,  were  it  not  for 
various  considerations,  one  turn  would  always  give  the  best 
result. 

I  have  found  that  it  is  preferable  to  use  in  these  experiments 
an  alternate  current  machine  giving  a  moderate  number  of  alter- 


398  .  NVENTION8  OF  NIKOLA   TESLA. 

nations  per  second  to  excite  the  induction  coil  for  charging  the 
Leyden  jar  which  discharges  through  the  primary — shown  dia- 
grammatically  in  Fig.  211, — as  in  such  case,  before  the  disrup- 
tive discharge  takes  place,  the  tube  or  bulb  is  slightly  excited  and 
the  formation  of  the  luminous  circle  is  decidedly  facilitated. 


FIG.  211. 

But  I  have  also  used  a  Wimshurst  machine  in  some  experi- 
ments. 

Prof.  J.  J.  Thomson's  view  of  the  phenomena  under  consid- 
eration seems  to  be  that  they  are  wholly  due  to  electro-magnetic 
action.  I  was,  at  one  time,  of  the  same  opinion,  but  upon  care- 
fully investigating  the  subject  I  was  led  to  the  conviction  that 
they  are  more  of  an  electrostatic  nature.  It  must  be  remem- 
bered that  in  these  experiments  we  have  to  deal  with  primary 
currents  of  an  enormous  frequency  or  rate  of  change  and  of  high 
potential,  and  that  the  secondary  conductor  consists  of  a  rarefied 


FIG.  212. 


gas,  and  that  under  such  conditions  electrostatic  effects  must  play 
an  important  part. 

In  support  of  my  view  I  will  describe  a  few  experiments  made 
by  me.  To  excite  luminosity  in  the  tube  it  is  not  absolutely 
necessary  that  the  conductor  should  be  closed.  For  instance,  if 


HIGH  FREQUENCY  AND  HIGH  POTENTIAL  CURRENTS.    399 

an  ordinary  exhausted  tube  (preferably  of  large  diameter)  be 
surrounded  by  a  spiral  of  thick  copper  wire  serving  as  the  prim- 
ary^ a  feebly  luminous  spiral  may  be  induced  in  the  tube,  roughly 
shown  in  Fig.  212.  In  one  of  these  experiments  a  curious  phe- 
nomenon was  observed  ;  namely,  two  intensely  luminous  circles, 
each  of  them  close  to  a  turn  of  the  primary  spiral,  were  formed 
inside  of  the  tube,  and  I  attributed  this  phenomenon  to  the  ex- 
istence of  nodes  on  the  primary.  The  circles  were  connected  by 
a  faint  luminous  spiral  parallel  to  the  primary  and  in  close  prox- 
imity to  it.  To  produce  this  effect  I  have  found  it  necessary  to 
strain  the  jar  to  the  utmost.  The  turns  of  the  spiral  tend  to 
close  and  form  circles,  but  this,  of  course,  would  be  expected, 
and  does  not  necessarily  indicate  an  electro-magnetic  effect ; 
whereas  the  fact  that  a  glow  can  be  produced  along  the  primary 
in  the  form  of  an  open  spiral  argues  for  an  electrostatic  effect. 


FIG.  213. 

In  using  Dr.  Lodge's  recoil  circuit,  the  electrostatic  action  is 
likewise  apparent.  The  arrangement  is  illustrated  in  Fig.  213. 
In  his  experiment  two  hollow  exhausted  tubes  H  H  were  slipped 
over  the  wires  of  the  recoil  circuit  and  upon  discharging  the  jar 
in  the  usual  manner  luminosity  was  excited  in  the  tubes. 

Another  experiment  performed  is  illustrated  in  Fig.  214.  In 
this  case  an  ordinary  lamp-bulb  was  surrounded  by  one  or  two 
turns  of  thick  copper  wire  P  and  the  luminous  circle  L  excited 
in  the  bulb  by  discharging  the  jar  through  the  primary.  The 
lamp-bulb  was  provided  with  a  tinfoil  coating  on  the  side  oppo- 
site to  the  primary  and  each  time  the  tinfoil  coating  was  con- 
nected to  the  ground  or  to  a  large  object  the  luminosity  of  the 
circle  was  considerably  increased.  This  was  evidently  due  to 
electrostatic  action. 

In  other  experiments  I  have  noted  that  when  the  primary 
touches  the  glass  the  luminous  circle  is  easier  produced  and  is 


400 


INVENTIONS  OF  NIKOLA  TESLA. 


more  sharply  defined ;  but  I  have  not  noted  that,  generally  speak- 
ing, the  circles  induced  were  very  sharply  defined,  as  Prof.  J.  J. 
Thomson  has  observed ;  on  the  contrary,  in  my  experiments  they 
were  broad  and  often  the  whole  of  the  bulb  or  tube  was  illumi- 
nated ;  and  in  one  case  I  have  observed  an  intensely  purplish 


FIG.  214. 


glow,  to  which  Prof.  J.  J.  Thomson  refers.  But  the  circles  were 
always  in  close  proximity  to  the  primary  and  were  considerably 
easier  produced  when  the  latter  was  very  close  to  the  glass,  much 
more  so  than  would  be  expected  assuming  the  action  to  be  elec- 


FIG.  215. 

tromagnetic  and  considering  the  distance ;  and  these  facts  speak 
for  an  electrostatic  effect. 

Furthermore  I  have  observed  that  there  is  a  molecular  bom- 
bardment in  the  plane  of  the  luminous  circle  at  right  angles  to 
the  glass — supposing  the  circle  to  be  in  the  plane  of  the  primary 


HIGH  FREQUENCY  AND  HIGH  POTENTIAL  CURRENTS.     401 

— this  bombardment  being  evident  from  the  rapid  heating  of  the 
glass  near  the  primary.  Were  the  bombardment  not  at  right 
angles  to  the  glass  the  heating  could  not  be  so  rapid.  If  there 
is  a  circumferential  movement  of  the  molecules  constituting  the 
luminous  circle,  I  have  thought  that  it  might  be  rendered  mani- 
fest by  placing  within  the  tube  or  bulb,  radially  to  the  circle,  a 
thin  plate  of  mica  coated  with  some  phosphorescent  material  and 
another  such  plate  tangentially  to  the  circle.  If  the  molecules 
would  move  circumferentially,  the  former  plate  would  be  ren- 
dered more  intensely  phosphorescent.  For  want  of  time  I  have, 
however,  not  been  able  to  perform  the  experiment. 

Another  observation  made  by  me  was  that  when  the  specific 
inductive  capacity  of  the  medium  between  the  primary  and 
secondary  is  increased,  the  inductive  effect  is  augmented.  This 
is  roughly  illustrated  in  Fig.  215.  In  this  case  luminosity  was 
excited  in  an  exhausted  tube  or  bulb  B  and  a  glass  tube  T  slipped 
between  the  primary  and  the  bulb,  when  the  effect  pointed  out 
was  noted.  Were  the  action  wholly  electromagnetic  no  change 
could  possibly  have  been  observed. 

I  have  likewise  noted  that  when  a  bulb  is  surrounded  by  a 
wire  closed  upon  itself  and  in  the  plane  of  the  primary,  the  for- 
mation of  the  luminous  circle  within  the  bulb  is  not  prevented. 
But  if  instead  of  the  wire  a  broad  strip  of  tinfoil  is  glued  upon 
the  bulb,  the  formation  of  the  luminous  band  was  prevented,  be- 
cause then  the  action  was  distributed  over  a  greater  surface.  The 
effect  of  the  closed  tinfoil  was  no  doubt  of  an  electrostatic  nature, 
for  it  presented  a  much  greater  resistance  than  the  closed  wire 
and  produced  therefore  a  much  smaller  electromagnetic  effect. 

Some  of  the  experiments  of  Prof.  J.  J.  Thomson  also  would 
seem  to  show  some  electrostatic  action.  For  instance,  in  the  ex- 
periment with  the  bulb  enclosed  in  a  bell  jar,  I  should  think 
that  when  the  latter  is  exhausted  so  far  that  the  gas  enclosed 
reaches  the  maximum  conductivity,  the  formation  of  the  circle 
in  the  bulb  and  jar  is  prevented  because  of  the  space  surrounding 
the  primary  being  highly  conducting ;  when  the  jar  is  further 
exhausted,  the  conductivity  of  the  space  around  the  primary 
diminishes  and  the  circles  appear  necessarily  first  in  the  bell  jar, 
as  the  rarefied  gas  is  nearer  to  the  primary.  But  were  the  in- 
ductive effect  very  powerful,  they  would  probably  appear  in  the 
bulb  also.  If,  however,  the  bell  jar  were  exhausted  to  the  high- 
est degree  they  would  very  likely  show  themselves  in  the  bulb 


403 


INVENTIONS  OF  NIKOLA  TE8LA. 


only,  that  is,  supposing  the  vacuous  space  to  be  non-conducting. 
On  the  assumption  that  in  these  phenomena  electrostatic  actions 
are  concerned  we  find  it  easily  explicable  why  the  introduction 
of  mercury  or  the  heating  of  the  bulb  prevents  the  formation  of 
the  luminous  band  or  shortens  the  after-glow ;  and  also  why  in 
some  cases  a  platinum  wire  may  prevent  the  excitation  of  the 
tube.  Nevertheless  some  of  the  experiments  of  Prof.  J.  J. 
Thomson  would  seem  to  indicate  an  electromagnetic  effect.  I 
may  add  that  in  one  of  my  experiments  in  which  a  vacuum  was 
produced  by  the  Torricellian  method,  I  was  unable  to  produce 
the  luminous  band,  but  this  may  have  been  due  to  the  weak  ex- 
citing current  employed. 

My  principal  argument  is  the  following  :  I  have  experiment- 
ally proved  that  if  the  same  discharge  which  is  barely  sufficient 
to  excite  a  luminous  band  in  the  bulb  when  passed  through  the 
primary  circuit  be  so  directed  as  to  exalt  the  electrostatic  induc- 
tive effect — namely,  by  converting  upwards — an  exhausted  tube, 
devoid  of  electrodes,  may  be  excited  at  a  distance  of  several  feet. 


SOME  EXPERIMENTS  ON  THE  ELECTRIC  DISCHARGE  IN  VACUUM  TUBES. 
BY  PROF.  J.  J.  THOMSON,  M.A.,  F.R.S. 

The  phenomena  of  vacuum  discharges  were,  Prof.  Thomson  said,  greatly 
simplified  when  their  path  was  wholly  gaseous,  the  complication  of  the  dark 
space  surrounding  the  negative  electrode,  and  the  stratifications  so  commonly 
observed  in  ordinary  vaciium  tubes,  being  absent.  To  produce  discharges  in 


FIG.  216. 


FIG.  2V, 


tubes  devoid  of  electrodes  was,  however,  not  easy  to  accomplish,  for  the  only 
available  means  of  producing  an  electromotive  force  in  the  discharge  circuit 
was  by  electro-magnetic  induction.  Ordinary  methods  of  producing  variable 
induction  were  valueless,  and  recourse  was  had  to  the  oscillatory  discharge  of  a 

1.  Abstract  of  a  paper  read  before  Physical  Society  of  London. 


HIGH  FREQUENCY  AND  HTGII  POTENTIAL  CURRENTS.     403 

Leyden  jar,  which  combines  the  two  essentials  of  a  current  whose  maximum 
value  is  enormous,  and  whose  rapidity  of  alternation  is  immensely  great.  The 
discharge  circuits,  which  may  take  the  shape  of  bulbs,  or  of  tubes  bent  in  the 
form  of  coils,  were  placed  in  close  proximity  to  glass  tubes  filled  with  mercury, 
which  formed  the  path  of  the  oscillatory  discharge.  The  parts  thus  corres- 
ponded to  the  windings  of  an  induction  coil,  the  vacuum  tubes  being  the  sec- 
ondary,  and  the  tubes  filled  with  mercury  the  primary.  In  such  an  apparatus 
the  Leyden  jar  need  not  be  large,  and  neither  primary  nor  secondary  need  have 
many  turns,  for  this  would  increase  the  self-induction  of  the  former,  and 
lengthen  the  discharge  path  in  the  latter.  Increasing  the  self-induction  of  the 
primary  reduces  the  E.  M.  F.  induced  in  the  secondary,  whilst  lengthening  the 
secondary  does  not  increase  the  E.  M.  F.  per  unit  length.  The  two  or  three 
turns,  as  shown  in  Fig.  216,  in  each,  were  found  to  be  quite  sufficient,  and,  on 
discharging  the  Leyden  jar  between  two  highly  polished  knobs  in  the  primary 


FIG.  218. 


FIG.  219. 


circuit,  a  plain  uniform  band  of  light  was  seen  to  pass  round  the  secondary. 
An  exhausted  bulb,  Fig.  217,  containing  traces  of  oxygen  was  plaeed  within  a 
primary  spiral  of  three  turns,  and,  on  passing  the  jar  discharge,  a  circle  of  light 
was  seen  within  the  bulb  inclose  proximity  to  the  primary  circuit,  accom- 
panied by  a  purplish  glow,  which  lasted  for  a  second  or  more.  On  heating  the 
bulb,  the  duration  of  the  glow  was  greatly  diminished,  and  it  could  be  in- 
stantly extinguished  by  the  presence  of  an  electro-magnet.  Another  exhausted 
bulb,  Fig.  218,  surrounded  by  a  primary  spiral,  was  contained  in  a  bell-jar, 
and  when  the  pressure  of  air  in  the  jar  was  about  that  of  the  atmosphere,  the 
secondary  discharge  occurred  in  the  bulb,  as  is  ordinarily  the  case.  On  ex- 
hausting the  jar,  however,  the  luminous  discharge  grew  fainter,  and  a  point 
was  reached  at  which  no  secondary  discharge  was  visible.  J  urther  exhaustion 
of  the  jar  caused  the  secondary  discharge  to  appear  outside  of  the  bulb.  The 
fact  of  obtaining  no  luminous  discharge,  either  in  the  bulb  or  jar,  the  author 


404  INVENTIONS  OF  NIKOLA  TESLA. 

could  only  explain  on  two  suppositions,  viz.:  that  under  the  conditions  then  ex- 
isting the  specific  inductive  capacity  of  the  gas  was  very  great,  or  that  a  dis- 
charge could  pass  without  being  luminous.  '1  he  author  had  also  .observed 
that  the  conductivity  of  a  vacuum  tube  without  electrodes  increased  as  the  pres- 
sure diminished,  until  a  certain  point  was  reached,  and  afterwards  diminished 
again,  thus  showing  that  the  high  resistance  of  a  nearly  perfect  vacuum  is  in 
no  way  due  to  the  presence  of  the  electrodes.  One  peculiarity  of  the  discharges 
was  their  local  nature,  the  rings  of  light  being  much  more  sharply  denned  than 
was  to  be  expected.  They  were  also  found  to  be  most  easily  produced  when 
the  chain  of  molecules  in  the  discharge  were  all  of  the  same  kind.  For  ex- 
ample, a  discharge  could  be  easily  sent  through  a  tube  many  feet  long,  but  the 
introduction  of  a  small  pellet  of  mercury  in  the  tube  stopped  the  discharge, 
although  the  conductivity  of  the  mercury  was  much  greater  than  that  of  the 
vacuum.  In  some  cases  he  had  noticed  that  a  very  fine  wire  placed  within  a 
tube,  on  the  side  remote  from  the  primary  circuit,  would  prevent  a  luminous 
discharge  in  that  tube. 

Pig.  219  shows  an  exhausted  secondary  coil  of  one  loop  containing  bulbs  ; 
the  discharge  passed  along  the  inner  side  of  the  bulbs,  the  primary  coils  being 
placed  within  the  secondary. 


1  In  The  Electrical  Engineer  of  August  12,  I  find  some  re- 
marks of  Prof.  J.  J.  Thomson,  which  appeared  originally  in  the 
London  Electrician  and  which  have  a  bearing  upon  some  experi- 
ments described  by  me  in  your  issue  of  July  1. 

I  did  not,  as  Prof.  J.  J.  Thomson  seems  to  believe,  misunder- 
stand his  position  in  regard  to  the  cause  of  the  phenomena 
considered,  but  I  thought  that  in  his  experiments,  as  well  as  in 
my  own,  electrostatic  effects  were  of  great  importance.  It  did 
not  appear,  from  the  meagre  description  of  his  experiments,  that 
all  possible  precautions  had  been  taken  to  exclude  these  effects. 
I  did  not  doubt  that  luminosity  could  be  excited  in  a  closed  tube 
when  electrostatic  action  is  completely  excluded.  In  fact,  at  the 
outset,  I  myself  looked  for  a  purely  electrodynamic  effect  and 
believed  that  I  had  obtained  it.  But  many  experiments  per- 
formed at  that  time  proved  to  me  that  the  electrostatic  effects 
were  generally  of  far  greater  importance,  and  admitted  of  a  more 
satisfactory  explanation  of  most  of  the  phenomena  observed. 

In  using  the  term  electrostatic  I  had  reference  rather  to  the 
nature  of  the  action  than  to  a  stationary  condition,  which  is  the 
usual  acceptance  of  the  term.  To  express  myself  more  clearly, 
I  will  suppose  that  near  a  closed  exhausted  tube  be  placed  a  small 
sphere  charged  to  a  very  high  potential.  The  sphere  would  act 
inductively  upon  the  tube,  and  by  distributing  electricity  over 
1.  Article  by  Mr.  Tesla  in  The  Electrical  Engineer,  N.  Y.,  August  26,  1891. 


HIGH  FREQUENCY  AND  HIGH  POTENTIAL  CURRENTS.     405 

the  same  would  undoubtedly  produce  luminosity  (if  the  potential 
be  sufficiently  high),  until  a  permanent  condition  would  be 
reached.  Assuming  the  tube  to  be  perfectly  well  insulated, 
there  would  be  only  one  instantaneous  flash  during  the  act  of 
distribution.  This  would  be  due  to  the  electrostatic  action 
simply. 

But  now,  suppose  the  charged  sphere  to  be  moved  at  short  in- 
tervals with  great  speed  along  the  exhausted  tube.  The  tube 
would  now  be  permanently  excited,  as  the  moving  sphere  would 
cause  a  constant  redistribution  of  electricity  and  collisions  of  the 
molecules  of  the  rarefied  gas.  We  would  still  have  to  deal  with 
an  electrostatic  eifect,  and  in  addition  an  electrodynamic  effect 
would  be  observed.  But  if  it  were  found  that,  for  instance,  the 
effect  produced  depended  more  on  the  specific  inductive  capa- 
city than  on  the  magnetic  permeability  of  the  medium — which 
would  certainly  be  the  case  for  speeds  incomparably  lower  than 
that  of  light — then  I  believe  I  would  be  justified  in  saying  that 
the  effect  produced  was  more  of  an  electrostatic  nature.  I  do 
not  mean  to  say,  however,  that  any  similar  condition  prevails  in 
the  case  of  the  discharge  of  a  Leyden  jar  through  the  primary, 
but  I  think  that  such  an  action  would  be  desirable. 

It  is  in  the  spirit  of  the  above  example  that  I  used  the  terms 
"  more  of  an  electrostatic  nature,"  and  have  investigated  the  in- 
fluence of  bodies  of  high  specific  inductive  capacity,  and  observed, 
for  instance,  the  importance  of  the  quality  of  glass  of  which  the 
tube  is  made.  I  also  endeavored  to  ascertain  the  influence  of  a 
medium  of  high  permeability  by  using  oxygen.  It  appeared 
from  rough  estimation  that  an  oxygen  tube  when  excited  under 
similar  conditions — that  is,  as  far  as  could  be  determined — gives 
more  light ;  but  this,  of  course,  may  be  due  to  many  causes. 

Without  doubting  in  the  least  that,  with  the  care  and  precau- 
tions taken  by  Prof.  J.  J.  Thomson,  the  luminosity  excited  was 
due  solely  to  electrodynamic  action,  I  would  say  that  in  many 
experiments  I  have  observed  curious  instances  of  the  ineffective- 
ness of  the  screening,  and  I  have  also  found  that  the  electritica. 
tion  through  the  air  is  often  of  very  great  importance,  and  may, 
in  some  cases,  determine  the  excitation  of  the  tube. 

In  his  original  communication  to  the  Electrician,  Prof.  J.  J. 
Thomson  refers  to  the  fact  that  the  luminosity  in  a  tube  near  a 
wire  through  which  a  Leyden  jar  was  discharged  was  noted  by 
Hittorf.  I  think  that  the  feeble  luminous  effect  referred  to  has 


406  INVENTIONS  OF  NIKOLA  TESLA. 

been  noted  by  many  experimenters,  but  in  my  experiments  the 
effects  were  much  more  powerful  than  those  usually  noted. 
The  following  is  the  communication1  referred  to  : — 


"Mr.  Tesla  seems  to  ascribe  the  effects  he  observed  to  electrostatic  action, 
and  I  have  no  doubt,  from  the  description  he  gives  of  his  method  of  conduct- 
ing his  experiments,  that  in  them  electrostatic  action  plays  a  very  important 
part.  He  seems,  however,  to  have  misunderstood  my  position  with  respect  to 
the  cause  of  these  discharges,  which  is  not,  as  he  implies,  that  luminosity  in 
tubes  without  electrodes  cannot  be  produced  by  electrostatic  action,  but  that  it 
can  also  be  produced  when  this  action  is  excluded.  As  a  matter  of  fact,  it  is 
very  much  easier  to  get  the  luminosity  when  these  electrostatic  effects  are 
operative  than  when  they  are  not.  As  an  illustration  of  this  I  may  mention 
that  the  first  experiment  I  tried  with  the  discharge  of  a  Leyden  jar  produced 
luminosity  in  the  tube,  but  it  was  not  until  after  six  weeks'  continuous  experi- 
menting that  I  was  able  to  get  a  discharge  in  the  exhausted  tube  which  I  was 
satisfied  was  due  to  what  is  ordinarily  called  electrodynamic  action.  It  is  ad- 
visable to  have  a  clear  idea  of  what  we  mean  by  electrostatic  action.  If, 
previous  to  the  discharge  of  the  jar,  the  primary  coil  is  raised  to  a  high  po- 
tential, it  will  induce  over  the  glass  of  the  tube  a  distribution  of  electricity. 
When  the  potential  of  the  primary  suddenly  falls,  this  electrification  will  re- 
distribute itself,  and  may  pass  through  the  rarefied  gas  and  produce  luminosity 
in  doing  so.  "Whilst  the  discharge  of  the  jar  is  going  on,  it  is  difficult,  and, 
from  a  theoretical  point  of  view,  undesirable,  to  separate  the  effect  into  parts, 
one  of  which  is  called  electrostatic,  the  other  electromagnetic  ;  what  we  can 
prove  is  that  in  this  case  the  discharge  is  not  such  as  would  be  produced  by 
electromotive  forces  derived  from  a  potential  function.  In  my  experiments  the 
primary  coil  was  connected  to  earth,  and,  as  a  further  precaution,  the  primary 
was  separated  from  the  discharge  tube  by  a  screen  of  blotting  paper,  moistened 
with  dilute  sulphuric  acid,  and  connected  to  earth.  Wet  blotting  paper  is  a 
sufficiently  good  conductor  to  screen  off  a  stationary  electrostatic  effect,  though 
it  is  not  a  good  enough  one  to  stop  waves  of  alternating  electromotive  intensity. 
When  showing  the  experiments  to  the  Physical  Society  I  could  not,  of  course, 
keep  the  tubes  covered  up,  but,  unless  my  memory  deceives  me,  I  stated  the 
precautions  which  had  b«en  taken  against  the  electrostatic  effect.  To  correct 
misapprehension  I  may  state  that  I  did  not  read  a  formal  paper  to  the  Society, 
my  object  being  to  exhibit  a  few  of  the  most  typical  experiments.  The  ac- 
count of  the  experiments  in  the  Electrician  was  from  a  reporter's  note,  and  was 
not  written,  or  even  read,  by  me.  I  have  now  almost  finished  writing  out,  and 
hope  very  shortly  to  publish,  an  account  of  these  and  a  large  number  of  allied 
experiments,  including  some  analogous  to  those  mentioned  by  Mr.  Tesla  on  the 
effect  of  conductors  placed  near  the  discharge  tube,  which  I  find,  in  some 
cases,  to  produce  a  diminution,  in  others  an  increase,  in  the  brightness  of  the 
discharge,  as  well  as  some  on  the  effect  of  the  presence  of  substances  of  large 
specific  inductive  capacity.  These  seem  to  me  to  admit  of  a  satisfactory  ex- 
planation, for  which,  however,  I  must  refer  to  my  paper." 

1.     Note  by  Prof.  J.  J.  Thomson  in  the  London  Electrician,  July  24,  1891. 


PART  III. 


MISCELLANEOUS  INVENTIONS  AND 
WRITINGS. 


CHAPTER  XXXIII. 

METHOD  OF  OBTAINING  DIRECT  FROM  ALTERNATING    CURRENTS. 

THIS  method  consists  in  obtaining  direct  from  alternating 
currents,  or  in  directing  the  waves  of  an  alternating  current  so  as 
to  produce  direct  or  substantially  direct  currents  by  developing 
or  producing  in  the  branches  of  a  circuit  including  a  source  of  al- 
ternating currents,  either  permanently  or  periodically,  and  by 
electric,  electro-magnetic,  or  magnetic  agencies,  manifestations  of 
energy,  or  what  may  be  termed  active  resistances  of  opposite 
electrical  character,  whereby  the  currents  or  current  waves  of  op- 
posite sign  will  be  diverted  through  different  circuits,  those  of 
one  sign  passing  over  one  branch  and  those  of  opposite  sign  over 
the  other. 

We  may  consider  herein  only  the  case  of  a  circuit  divided  into 
two  paths,  inasmuch  as  any  further  subdivision  involves  merely 
an  extension  of  the  general  principle.  Selecting,  then,  any  cir- 
cuit through  which  is  flowing  an  alternating  current,  Mr.  Tesla 
divides  such  circuit  at  any  desired  point  into  two  branches  or 
paths.  In  one  of  these  paths  he  inserts  some  device  to  create 
an  electromotive  force  counter  to  the  waves  or  impulses  of  cur- 
rent of  one  sign  and. a  similar  device  in  the  other  branch  which 
opposes  the  waves  of  opposite  sign.  Assume,  for  example,  that 
these  devices  are  batteries,  primary  or  secondary,  or  continuous 
current  dynamo  machines.  The  waves  or  impulses  of  opposite 
direction  composing  the  main  current  have  a  natural  tendency  to 
divide  between  the  two  branches ;  but  by  reason  of  the  opposite 
electrical  character  or  effect  of  the  two  branches,  one  will  offer 
an  easy  passage  to  a  current  of  a  certain  direction,  while  the  other 
will  offer  a  relatively  high  resistance  to  the  passage  of  the  same 
current.  The  result  of  this  disposition  is,  that  the  waves  of  cur- 
rent of  one  sign  will,  partly  or  wholly,  pass  over  one  of  the  paths 
or  branches,  while  those  of  the  opposite  sign  pass  over  the  other. 
There  may  thus  be  obtained  from  an  alternating  current  two  or 
more  direct  currents  without  the  employment  of  any  commutator 


410  INVENTIONS  OF  NIKOLA  TESLA. 

such  as  it  has  been  heretofore  regarded  as  necessary  to  use.  The 
current  in  either  branch  may  be  used  in  the  same  way  and  for 
the  same  purposes  as  any  other  direct  current — that  is,  it  may  be 
made  to  charge  secondary  batteries,  energize  electro-magnets,  or 
for  any  other  analogous  purpose. 

Fig.  220  represents  a  plan  of  directing  the  alternating  currents 
by  means  of  devices  purely  electrical  in  character.  Figs.  221, 
222,  223,  224,  225,  and  226  are  diagrams  illustrative  of  other 
ways  of  carrying  out  the  invention. 

In  Fig.  220,  A  designates  a  generator  of  alternating  currents, 
and  B  B  the  main  or  line  circuit  therefrom.  At  any  given  point 
in  this  circuit  at  or  near  which  it  is  desired  to  obtain  direct  cur- 
rents, the  circuit  B  is  divided  into  two  paths  or  branches  c  D.  In 
each  of  these  branches  is  placed  an  electrical  generator,  which 
for  the  present  we  will  assume  produces  direct  or  continuous  cur- 


FIG.  220. 

rents.  The  direction  of  the  current  thus  produced  is  opposite  in 
one  branch  to  that  of  the  current  in  the  other  branch,  or,  con- 
sidering the  two  branches  as  forming  a  closed  circuit,  the  gene- 
rators E  F  are  connected  up  in  series  therein,  one  generator  in 
each  part  or  half  of  the  circuit.  The  electromotive  force  of  the 
current  sources  E  and  F  may  be  equal  to  or  higher  or  lower  than 
the  electromotive  forces  in  the  branches  c  D,  or  between  the  points 
x  and  Y  of  the  circuit  B  B.  If  equal,  it  is  evident  that  current 
waves  of  one  sign  will  be  opposed  in  one  branch  and  assisted  in 
the  other  to  such  an  extent  that  all  the  waves  of  one  sign  will 
pass  over  one  branch  and  those  of  opposite  sign  over  the  other. 
If,  on  the  other  hand,  the  electromotive  force  of  the  sources  E  F 
be  lower  than  that  between  x  and  Y,  the  currents  in  both 
branches  will  be  alternating,  but  the  waves  of  one  sign  will  pre- 
ponderate. One  of  the  generators  or  sources  of  current  E  or  F 
may  be  dispensed  with ;  but  it  is  preferable  to  employ  both,  if 


OBTAINING  DIRECT  FROM  ALTERNATING  CURRENTS.    411 

they  offer  an  appreciable  resistance,  as  the  two  brandies  will  be 
thereby  better  balanced.  The  translating  or  other  devices  to  be 
acted  upon  by  the  current  are  designated  by  the  letters  G,  and 
they  are  inserted  in  the  branches  c  D  in  any  desired  manner  ;  but 
in  order  to  better  preserve  an  even  balance  between  the  branches 
due  regard  should,  of  course,  be  had  to  the  number  and  character 
of  the  devices. 

Figs.  221,  222,  223,  and  224  illustrate  what  may  termed  "elec- 
tro-magnetic" devices  for  accomplishing  a  similar  result — that  is 
to  say,  instead  of  producing  directly  by  a  generator  an  electro- 
motive force  in  each  branch  of  the  circuit,  Mr.  Tesla  establishes 
a  field  or  fields  of  force  and  leads  the  branches  through  the  same 
in  such  manner  that  an  active  opposition  of  opposite  effect  or  di- 
rection will  be  developed  therein  by  the  passage,  or  tendency  to 
pass,  of  the  alternations  of  current.  In  Fig.  221,  for  example,  A  is 


FIG.  221. 

the  generator  of  alternating  currents,  B  B  the  line  circuit,  and  c  D 
the  branches  over  which  the  alternating  currents  are  directed.  In 
each  branch  is  included  the  secondary  of  a  transformer  or  induc- 
tion coil,  which,  since  they  correspond  in  their  functions  to  the 
batteries  of  the  previous  figure,  are  designated  by  the  letters  E  F. 
The  primaries  H  H'  of  the  induction  coils  or  transformers  are 
connected  either  in  parallel  or  series  with  a  source  of  direct  or 
continuous  currents  i,  and  the  number  of  convolutions  is  so  cal- 
culated for  the  strength  of  the  current  from  i  that  the  cores  J  j' 
will  be  saturated.  The  connections  are  such  that  the  conditions 
in  the  two  transformers  are  of  opposite  character — that  is  to  say, 
the  arrangement  is  such  that  a  current  wave  or  impulse  corres- 
ponding in  direction  with  that  of  the  direct  current  in  one  pri- 
mary, as  H,  is  of  opposite  direction  to  that  in  the  other  primary  H'. 
It  thus  results  that  while  one  secondary  offers  a  resistance  or  op- 


412 


INVENTIONS  OF  NIKOLA  TESLA. 


position  to  the  passage  through  it  of  a  wave  of  one  sign,  the  other 
secondary  similarly  opposes  a  wave  of  opposite  sign.  In  conse- 
quence, the  waves  of  one  sign  will,  to  a  greater  or  less  extent,  pass 
by  way  of  one  branch,  while  those  of  opposite  sign  in  like  man- 
ner pass  over  the  other  branch. 

In  lieu  of  saturating  the  primaries  by  a  source  of  continuous 
current,  we  may  include  the  primaries  in  the  branches  c  D,  re- 
spectively, and  periodically  short-circuit  by  any  suitable  mechani- 
cal devices — such  as  an  ordinary  revolving  commutator — their 
secondaries.  It  will  be  understood,  of  course,  that  the  rotation 
and  action  of  the  commutator  must  be  in  synchronism  or  in 
proper  accord  with  the  periods  of  the  alternations  in  order  to 
secure  the  desired  results.  Such  a  disposition  is  represented 


FIG.  222. 

diagrammatically  in  Fig.  222.  Corresponding  to  the  previous 
figures,  A  is  the  generator  of  alternating  currents,  B  B  the  line, 
and  c  D  the  two  branches  for  the  direct  currents.  In  branch  c 
are  included  two  primary  coils  E  E',  and  in  branch  D  are  two 
similar  primaries  F  F'  The  corresponding  secondaries  for  these 
coils  and  which  are  on  the  same  subdivided  cores  j  or  j',  are  in 
circuits  the  terminals  of  which  connect  to  opposite  segments  K 
K',  and  L  i/,  respectively,  of  a  commutator.  Brushes  b  I  bear 
upon  the  commutator  and  alternately  short-circuit  the  plates  K 
and  K',  and  L  and  L',  through  a  connection  c.  It  is  obvious  that 
either  the  magnets  and  commutator,  or  the  brushes,  may  revolve. 
The  operation  will  be  understood  from  a  consideration  of  the 
effects  of  closing  or  short-circuiting  the  secondaries.  For  ex- 
ample, if  at  the  instant  when  a  given  wave  of  current  passes,  one 


OBTAINING  DIRECT  FROM  ALTERNATING  CURRENTS.    418 

set  of  secondaries  be  short-circuited,  nearly  all  the  current  flows 
through  the  corresponding  primaries ;  but  the  secondaries  of  the 
other  branch  being  open-circuited,  the  self-induction  in  the 
primaries  is  highest,  and  hence  little  or  no  current  will  pass 
through  that  branch.  If,  as  the  current  alternates,  the  second- 
aries of  the  two  branches  are  alternately  short-circuited,  the 
result  will  be  that  the  currents  of  one  sign  pass  over  one  branch 
and  those  of  the  opposite  sign  over  the  other.  The  disadvan- 
tages of  this  arrangement,  which  would  seem  to  result  from  the 
employment  of  sliding  contacts,  are  in  reality  very  slight,  inas- 
much as  the  electromotive  force  of  the  secondaries  may  be  made 
exceedingly  low,  so  that  sparking  at  the  brushes  is  avoided. 

Fig.  223  is  a  diagram,  partly  in  section,  of  another  plan  of 
carrying  out  the  invention.  The  circuit  B  in  this  case  is  divided, 
as  before,  and  each  branch  includes  the  coils  of  both  the  fields 


FIG.  223. 

and  revolving  armatures  of  two  induction  devices.  The  arma- 
tures o  P  are  preferably  mounted  on  the  same  shaft,  and  are  ad- 
justed relatively  to  one  another  in  such  manner  that  when  the 
self-induction  in  one  branch,  as  c.  is  maximum,  in  the  other  branch 
D  it  is  minimum.  The  armatures  are  rotated  in  synchronism  with 
the  alternations  from  the  source  A.  The  winding  or  position 
of  the  armature  coils  is  such  that  a  current  in  a  given  direction 
passed  through  both  armatures  would  establish  in  one,  poles  simi- 
lar to  those  in  the  adjacent  poles  of  the  field,  and  in  the  other, 
poles  unlike  the  adjacent  field  poles,  as  indicated  by  n  n  s  s  in 
the  diagram.  If  the  like  poles  are  presented,  as  shown  in  cir- 
cuit D,  the  condition  is  that  of  a  closed  secondary  upon  a  primary, 
or  the  position  of  least  inductive  resistance ;  hence  a  given  alter- 
nation of  current  will  pass  mainly  through  D.  A  half  revolution 
of  the  armatures  produces  an  opposite  effect  and  the  succeeding 


414 


INVENTIONS  OF  NIKOLA  TE8LA. 


current  impulse  passes  through  c.  Using  this  figure  as  an  illus- 
tration, it  is  evident  that  the  fields  N  M  may  be  permanent  mag- 
nets or  independently  excited  and  the  armatures  o  P  driven,  as  in 
the  present  case,  so  as  to  produce  alternate  currents,  which  will 
set  up  alternately  impulses  of  opposite  direction  in  the  two 
branches  D  c,  which  in  such  case  would  include  the  armature  cir- 
cuits and  translating  devices  only. 

In  Fig.  224  a  plan  alternative  with  that  shown  in  Fig.  222  is 
illustrated.  In  the  previous  case  illustrated,  each  branch  c  and  D 
contained  one  or  more  primary  coils,  the  secondaries  of  which 
were  periodically  short  circuited  in  synchronism  with  the  alter- 
nations of  current  from  the  main  source  A,  and'  for  this  purpose 
a  commutator  was  employed.  The  latter  may,  however,  be  dis- 
pensed with  and  an  armature  with  a  closed  coil  substituted. 

Referring  to  Fig.  224  in  one  of  the  branches,  as  c,  are  two  coils 


FIG.  224. 

M',  wound  on  laminated  cores,  and  in  the  other  branches  D  are 
similar  coils  N'.  A  subdivided  or  laminated  armature  o7,  carry- 
ing a  closed  coil  R',  is  rotatably  supported  between  the  coils  M'  N', 
as  shown.  In  the  position  shown — that  is,  with  the  coil  K'  paral- 
lel with  the  convolutions  of  the  primaries  N'  M' — practically  the 
whole  current  will  pass  through  branch  D,  because  the  self-in- 
duction in  coils  M'  M'  is  maximum.  If,  therefore,  the  armature 
and  coil  be  rotated  at  a  proper  speed  relatively  to  the  periods  or 
alternations  of  the  source  A,  the  same  results  are  obtained  as  in 
the  case  of  Fig.  222. 

Fig.  225  is  an  instance  of  what  may  be  called,  in  distinction  to 
the  others,  a  "  magnetic  "  means  of  securing  the  result,  v  and 
w  are  two  strong  permanent  magnets  provided  with  armatures 
v'  w',  respectively.  The  armatures  are  made  of  thin  laminae  of 
soft  iron  or  steel,  and  the  amount  of  magnetic  metal  which  they 


OBTAINING  DIRECT  FROM  ALTERNATING  CURRENTS.     4ir> 

contain  is  so  calculated  that  they  will  be  fully  or  nearly  saturated 
by  the  magnets.  Around  the  armatures  are  coils  E  F,  contained, 
respectively,  in  the  circuits  c  and  D.  The  connections  and  elec- 
trical conditions  in  this  case  are  similar  to  those  in  Fig.  221, 
except  that  the  current  source  of  i,  Fig.  221,  is  dispensed  with 
and  the  saturation  of  the  core  of  coils  E  F  obtained  froth  the  per- 
manent magnets. 

The  previous  illustrations  have  all  shown  the  two  branches  or 
paths  containing  the  translating  or  induction  devices  as  in  deriva- 
tion one  to  the  other ;  but  this  is  not  always  necessary.  For 
example,  in  Fig.  226,  A  is  an  alternating-current  generator;  B  B, 
the  line  wires  or  circuit.  At  any  given  point  in  the  circuit  let 
us  form  two  paths,  as  D  D',  and  at  another  point  two  paths,  as  c 
c' '.  Either  pair  or  group  of  paths  is  similar  to  the  previous  dis- 


FIG.  225. 

positions  with  the  electrical  source  or  induction  device  in  one 
branch  only,  while  the  two  groups  taken  together  form  the 
obvious  equivalent  of  the  cases  in  which  an  induction  device  or 
generator  is  included  in  both  branches.  In  one  of  the  paths,  as 
D,  are  included  the  devices  to  be  operated  by  the  current.  In 
the  other  branch,  as  D',  is  an  induction  device  that  opposes  the 
current  impulses  of  one  direction  and  directs  them  through  the 
branch  D.  So,  also,  in  branch  c  are  translating  devices  o,  and  in 
branch  c'  an  induction  device  or  its  equivalent  that  diverts 
through  c  impulses  of  opposite  direction  to  those  diverted  by  the 
device  in  branch  D'.  The  diagram  shows  a  special  form  of  in- 
duction device  for  this  purpose,  .r  /  are  the  cores,  formed  with 
pole-pieces,  upon  which  are  wound  the  coils  M  N.  Between  these 
pole-pieces  are  mounted  at  right  angles  to  one  another  the  mag- 
netic armatures  o  P,  preferably  mounted  on  the  same  shaft  and 


416 


INVENTIONS  OF  NIKOLA  TESLA. 


designed  to  be  rotated  in  synchronism  with  the  alternations  of 
current.  When  one  of  the  armatures  is  in  line  with  the  poles  or 
in  the  position  occupied  by  armature  p,  the  magnetic  circuit  of 
the  induction  device  is  practically  closed ;  hence  there  will  be 
the  greatest  opposition  to  the  passage  of  a  current  through  coils 
N  N.  The  alternation  will  therefore  pass  by  way  of  branch  D. 
At  the  same  time,  the  magnetic  circuit  of  the  other  induction 
device  being  broken  by  the  position  of  the  armature  o,  there  will 
be  less  opposition  to  the  current  in  coils  M,  which  will  shunt  the 
current  from  branch  c.  A  reversal  of  the  current  being  attended 
by  a  shifting  of  the  armatures,  the  opposite  effect  is  produced. 

Other  modifications  of  these  methods  are  possible,  but  need 
not  be  pointed  out.     In  all  these  plans,  it  will  be  observed,  there 


PIG 


is  developed  in  one  or  all  of  these  branches  of  a  circuit  from  a 
source  of  alternating  currents,  an  active  (as  distinguished  from  a 
dead)  resistance  or  opposition  to  the  currents  of  one  sign,  for  the 
purpose  of  diverting  the  currents  of  that  sign  through  the  other 
or  another  path,  but  permitting  the  currents  of  opposite  sign  to 
pass  without  substantial  opposition. 

Whether  the  division  of  the  currents  or  waves  of  current  of 
opposite  sign  be  effected  with  absolute  precision  or  not  is  imma- 
terial, since  it  will  be  sufficient  if  the  waves  are  only  partially 
diverted  or  directed,  for  in  such  case  the  preponderating  influence 
in  each  branch  of  the  circuit  of  the  waves  of  one  sign  secures 
the  same  practical  results  in  many  if  not  all  respects  as  though 
the  current  were  direct  and  continuous. 


OBTAINING  DIRECT  FROM  ALTERNATING  CURRENTS.    417 

An  alternating  and  a  direct  current  have  been  combined  so  that 
the  waves  of  one  direction  or  sign  were  partially  or  wholly  over- 
come by  the  direct  current ;  but  by  this  plan  only  one  set  of  al- 
ternations are  utilized,  whereas  by  the  system  just  described  the 
entire  current  is  rendered  available.  By  obvious  applications  of 
this  discovery  Mr.  Tesla  is  enabled  to  produce  a  self -exciting  al- 
ternating dynamo,  or  to  operate  direct  current  meters  on  alter- 
nating-current circuits  or  to  run  various  devices — such  as  arc  lamps 
— by  direct  currents  in  the  same  circuit  with  incandescent  lamps 
or  other  devices  operated  by  alternating  currents. 

It  will  be  observed  that  if  an  intermittent  counter  or  opposing 
force  be  developed  in  the  branches  of  the  circuit  and  of  higher 
electromotive  force  than  that  of  the  generator,  an  alternating 
current  will  result  in  each  branch,  with  the  waves  of  one  sign 
preponderating,  while  a  constantly  or  uniformly  acting  oppo- 
sition in  the  branches  of  higher  electromotive  force  than  the 
generator  would  produce  a  pulsating  current,  which  conditions 
would  be,  under  some  circumstances,  the  equivalent  of  those  de- 
scribed. 


CHAPTER  XXXIY. 

CONDENSERS  WITH  PLATES  IN  OIL. 

IN  experimenting  with  currents  of  high  frequency  and  high 
potential,  Mr.  Tesla  has  found  that  insulating  materials  such  as 
glass,  mica,  and  in  general  those  bodies  which  possess  the  highest 
specific  inductive  capacity,  are  inferior  as  insulators  in  such  de- 
vices when  currents  of  the  kind  described  are  employed  compared 
with  those  possessing  high  insulating  power,  together  with  a  smaller 
specific  inductive  capacity ;  and  he  has  also  found  that  it  is  very  de- 
sirable to  exclude  all  gaseous  matter  from  the  apparatus,  or  any  ac- 


FIG.  227. 


FIG.  228. 


cess  of  the  same  to  the  electrified  surfaces,  in  order  to  prevent  heat- 
ing by  molecular  bombardment  and  the  loss  or  injury  consequent 
thereon.  He  has  therefore  devised  a  method  to  accomplish  these 
results  and  produce  highly  efficient  and  reliable  condensers,  by 
using  oil  as  the  dielectric1.  The  plan  admits  of  a  particular  con- 

1.  Mr.  Tesla's  experiments,  as  the  careful  reader  of  his  three  lectures  will 
perceive,  have  revealed  a  very  important  fact  which  is  taken  advantage  of  in 
this  invention.  Namely,  he  has  shown  that  in  a  condenser  a  considerable 
amount  of  energy  may  be  wasted,  and  the  condenser  may  break  down  merely 
because  gaseous  matter  is  present  between  the  surfaces.  A  number  of  experi- 
ments are  described  in  the  lectures,  which  bring  out  this  fact  forcibly  and  serve 
as  a  guide  in  the  operation  of  high  tension  apparatus.  But  besides  bearing 
upon  this  point,  these  experiments  also  throw  a  light  upon  investigations  of  a 
purely  scientific  nature  and  explain  now  the  lack  of  harmony  among  the  ob- 
servations of  various  investigators.  Mr.  Tesla  shows  that  in  a  fluid  such  as  oil 
the  losses  are  very  small  as  compared  with  those  incurred  in  a  gas. 


. 


CONDENSERS  WITH  PL  A  TEN  IN  OIL.  419 

struction  of  condenser,  in  whicli  the  distance  between  the  plates 
is  adjustable,  and  of  which  he  takes  advantage. 

In  the  accompanying  illustrations,  Fig.  227  is  a  section  of  a 
condenser  constructed  in  accordance  with  this  principle  and  hav- 
ing stationary  plates ;  and  Fig.  228  is  a  similar  view  of  a  condenser 
with  adjustable  plates. 

Any  suitable  box  or  receptacle  A  may  be  used  to  contain  the 
plates  or  armatures.  These  latter  are  designated  by  B  and  c  and 
are  connected,  respectively,  to  terminals  i>  and  E,  which  pass  out 
through  the  sides  of  the  case.  The  plates  ordinarily  are  separated 
by  strips  of  porous  insulating  material  F,  which  are  used  merely 
for  the  purpose  of  maintaining  them  in  position.  The  space 
within  the  can  is  filled  with  oil  G.  Such  a  condenser  will  prove 
highly  efficient  and  will  not  become  heated  or  permanently  in- 
jured. 

In  many  cases  it  is  desirable  to  vary  or  adjust  the  capacity  of 
a  condenser,  and  this  is  provided  for  by  securing  the  plates  to  ad- 
justable supports — as,  for  example,  to  rods  n — passing  through 
stuffing  boxes  K  in  the  sides  of  case  A  and  furnished  with  nuts  L, 
the  ends  of  the  rods  being  threaded  for  engagement  with  the 
nuts. 

It  is  well  known  that  oils  possess  insulating  properties,  and  it 
it  has  been  a  common  practice  to  interpose  a  body  of  oil  between 
two  conductors  for  purposes  of  insulation ;  but  Mr.  Tesla  be- 
lieves he  has  discovered  peculiar  properties  in  oils  which  ren- 
der them  very  valuable  in  this  particular  form  of  device. 


CHAPTER  XXXY. 

ELECTROLYTIC    REGISTERING   METER. 

AN  ingenious  form  of  electrolytic  meter  attributable  to  Mr. 
Tesla  is  one  in  which  a  conductor  is  immersed  in  a  solution,  so 
arranged  that  metal  may  be  deposited  from  the  solution  or  taken 
away  in  such  a  manner  that  the  electrical  resistance  of  the  con- 
ductor is  varied  in  a  definite  proportion  to  the  strength  of  the 
current  the  energy  of  which  is  to  be  computed,  whereby  this 
variation  in  resistance  serves  as  a  measure  of  the  energy  and  also 
may  actuate  registering  mechanism,  whenever  the  resistance 
rises  above  or  falls  below  certain  limits. 

In  carrying  out  this  idea  Mr.  Tesla  employs  an  electroly- 
tic cell,  through  which  extend  two  conductors  parallel  and 
in  close  proximity  to  each  other.  These  conductors  he  connects 
in  series  through  a  resistance,  but  in  such  manner  that  there  is 
an  equal  difference  of  potential  between  them  throughout  their 
entire  extent.  The  free  ends  or  terminals  of  the  conductors'are 
connected  either  in  series  in  the  circuit  supplying  the  current  to 
the  lamps  or  other  devices,  or  in  parallel  to  a  resistance  in  the 
circuit  and  in  series  with  the  current  consuming  devices.  Under 
such  circumstances  a  current  passing  through  the  conductors 
establishes  a  difference  of  potential  between  them  which  is  pro- 
portional to  the  strength  of  the  current,  in  consequence  of  which 
there  is  a  leakage  of  current  from  one  conductor  to  the  other 
across  the  solution.  The  strength  of  this  leakage  current  is  pro- 
portional to  the  difference  of  potential,  and,  therefore,  in  propor- 
tion to  the  strength  of  the  current  passing  through  the  conductors. 
Moreover,  as  there  is  a  constant  difference  of  potential  between 
the  two  conductors  throughout  the  entire  extent  that  is  exposed 
to  the  solution,  the  current  density  through  such  solution  is  the 
same  at  all  corresponding  points,  and  hence  the  deposit  is  uni- 
form along  the  whole  of  one  of  the  conductors,  while  the  metal 
is  taken  away  uniformly  from  the  other.  The  resistance  of  one 
conductor  is  by  this  means  diminished,  while  that  of  the  other  is 


ELECTROLYTIC  REGISTERING  METER. 


421 


increased,  both  in  proportion  to  the  strength  of  the  current  pass- 
ing through  the  conductors.  From  sucli  variation  in  the  resis- 
tance of  either  or  both  of  the  conductors  forming  the  positive 
and  negative  electrodes  of  the  cell,  the  current  energy  expended 
may  be  readily  computed.  Figs.  229  and  230  illustrate  two 
forms  of  such  a  meter. 

In  Fig.  229  G  designates  a  direct-current  generator.  L  L  are 
the  conductors  of  the  circuit  extending  therefrom.  A  is  a  tube 
of  glass,  the  ends  of  which  are  scaled,  as  by  means  of  in- 
sulating plugs  or  caps  B  B.  c  c'  are  two  conductors  extending 
through  the  tube  A,  their  ends  passing  out  through  the  plugs  B  to 


FIG.  229. 


terminals  thereon.  These  conductors  may  be  corrugated  or 
formed  in  other  proper  ways  to  offer  the  desired  electrical  resis- 
tance. K  is  a  resistance  connected  in  series  witli  the  two  con- 
ductors c  c',  which  by  their  free  terminals  are  connected  up  in 
circuit  with  one  of  the  conductors  L. 

The  method  of  using  this  device  and  computing  by  means 
thereof  the  energy  of  the  current  will  be  readily  understood. 
First,  the  resistances  of  the  two  conductors  c  c',  respectively,  are 
accurately  measured  and  noted.  Then  a  known  current  is  passed 
through  the  instrument  for  a -given  time,  and  by  a  second  meas- 
urement the  increase  and  diminution  of  the  resistances  of  the  two 
conductors  are  respectively  taken.  From  these  data  the  constant  is 


422 


INVENTIONS  OF  NIKOLA  TKSLA. 


obtained — that  is  to  say,  for  example,  the  increase  of  resistance  of 
one  conductor  or  the  diminution  of  the  resistance  of  the  other  per 
lamp  hour.  These  two  measurements  evidently  serve  as  a  check, 
since  the  gain  of  one  conductor  should  equal  the  loss  of  the  other. 
A  further  check  is  afforded  by  measuring  both  wires  in  series  with 
the  resistance,  in  which  case  the  resistance  of  the  whole  should 
remain  constant. 

In  Fig.  230  the  conductors  c  c'  are  connected  in  parallel,  the 
current  device  at  x  passing  in  one  branch  iirst  through  a  resis- 
tance R'  and  then  through  conductor  c,  while  on  the  other  branch 
it  passes  iirst  through  conductor  c',  and  then  through  resistance 


FIG.  280. 


R".  The  resistances  R'  R"  are  equal,  as  also  are  the  resistances  of 
the  conductors  c  c'.  It  is,  moreover,  preferable  that  the  respective 
resistances  of  the  conductors  c  c'  should  be  a  known  and  con- 
venient fraction  of  the  coils  or  resistances  R'  R".  It  will  be  ob- 
served that  in  the  arrangement  shown  in  Fig.  230  there  is  a  constant 
potential  difference  between  the  two  conductors  c  c'  throughout 
their  entire  length. 

It  will  be  seen  that  in  both  cases  illustrated,  the  proportionality 
of  the  increase  or  decrease  of  resistance  to  the  current  strength 
will  always  be  preserved,  for  what  one  conductor  gains  the  other 
loses,  and  the  resistances  of  the  conductors  c  c'  being  small  as 


ELECTROLYTIC  REGISTERING  METER.  423 

compared  with  the  resistances  in  series  with  them.  It  will  be 
understood  that  after. each  measurement  or  registration  of  a  given 
variation  of  resistance  in  one  or  both  conductors,  the  direction  of 
the  current  should -be  changed  or  the  instrument  reversed,  so  that 
the  deposit  will  be  taken  from  the  conductor  which  has  gained 
and  added  to  that  which  has  lost.  This  principle  is  capable  of 
many  modifications.  For  instance,  since  there  is  a  section  of  the 
circuit — to  wit,  the  conductor  c  or  c' — that  varies  in  resistance  in 
proportion  to  the  current  strength,. such  variation  maybe  utilized, 
as  is  done  in  many  analogous  cases,  to  effect  the  operation  of 
various  automatic  devices,  such  as  registers.  It  is  better,  how- 
ever, for  the  sake  of  simplicity  to  compute  the  energy  by  meas- 
urements of  resistance. 

The  chief  advantages  of  this  arrangement  are,  first,  that  it  is 
possible  to  read  off  directly  the  amount  of  the  energy  expended 
by  means  of  a  properly  constructed  ohm-meter  and  without  re- 
sorting to  weighing  the  deposit ;  secondly  it  is  not  necessary  to 
employ  shunts,  for  the  whole  of  the  current  to  be  measured  may 
be  passed  through  the  instrument ;  third,  the  accuracy  of  the  in- 
strument and  correctness  of  the  indications  are  but  slightly  af- 
fected by  changes  in  temperature.  It  is  also  said  that  such  meters 
have  the  merit  of  superior  economy  and  compactness,  as  well  as 
of  cheapness  in  construction.  Electrolytic  meters  seem  to  need 
every  auxiliary  advantage  to  make  them  permanently  popular  and 
successful,  no  matter  how  much  ingenuity  may  be  shown  in  their 
design. 


CHAPTEK   XXXVI. 

THERMO-MAGNETIC  MOTORS  AND  PYRO-MAGNETIC  GENERATORS. 

No  electrical  inventor  of  the  present  day  dealing  with  the 
problems  of  light  and  power  considers  that  he  has  done  himself 
or  his  opportunities  justice  until  he  has  attacked  the  subject  of 
thermo-magiietism.  As  far  back  as  the  beginning  of  the  seven- 
teenth century  it  was  shown  by  Dr.  William  Gilbert,  the  father 
of  modern  electricity,  that  a  loadstone  or  iron  bar  when  heated 
to  redness  loses  its  magnetism ;  and  since  that  time  the  influence 
of  heat  on  the  magnetic  metals  has  been  investigated  frequently, 
though  not  with  any  material  or  practical  result. 

For  a  man  of  Mr.  Tesla's  inventive  ability,  the  problems  in 
this  field  have  naturally  had  no  small  fascination,  and  though  he 
has  but  glanced  at  them,  it  is  to  be  hoped  he  may  find  time  to 
pursue  the  study  deeper  and  further.  For  such  as  he,  the  in- 
vestigation must  undoubtedly  bear  fruit.  Meanwhile  he  has 
worked  out  one  or  two  operative  devices  worthy  of  note.1  He 
obtains  mechanical  power  by  a  reciprocating  action  resulting 
from  the  joint  operations  of  heat,  magnetism,  and  a  spring  or 
weight  or  other  force — that  is  to  say  he  subjects  a  body  magnet- 
ized by  induction  or  otherwise  to  the  action  of  heat  until  the 
magnetism  is  sufficiently  neutralized  to  allow  a  weight  or  spring 
to  give  motion  to  the  body  and  lessen  the  action  of  the  heat,  so 
that  the  magnetism  may  be  sufficiently  restored  to  move  the 

1.  It  will,  of  course,  be  inferred  from  the  nature  of  these  devices  that  the 
vibration  obtained  in  this  manner  is  very  slow  owing  to  the  inability  of  the 
iron  to  follow  rapid  changes  in  temperature.  In  an  interview  with  Mr.  Tesla 
on  this  subject,  the  compiler  learned  of  an  experiment  which  will  interest 
students.  A  simple  horseshoe  magnet  is  taken  and  a  piece  of  sheet  iron  bent  in 
the  form  of  an  L  is  brought  in  contact  with  one  of  the  poles  and  placed  in 
such  a  position  that  it  is  kept  in  the  attraction  of  the  opposite  pole  delicately 
suspended.  A  spirit  lamp  is  placed  under  the  sheet  iron  piece  and  when  the 
iron  is  heated  to  a  certain  temperature  it  is  easily  set  in  vibration  oscillating  as 
rapidly  as  400  to  500  times  a  minute.  The  experiment  is  very  easily  per- 
formed and  is  interesting  principally  on  account  of  the  very  rapid  rate  of 
vibration. 


TlIK  II  MO- MAGNETISM  AND  PYRO  MAGNKTIHM. 


4-25 


body  in  tlie  opposite  direction,  and  again  subject  the  same  to  the 
demagnetizing  power  of  the  heat. 

Use  is  made  of  either  an  electro-magnet  or  a  permanent  mag- 
net, and  the  heat  is  directed  against  a  body  that  is  magnetized 
by  induction,  rather  than  directly  against  a  permanent  magnet, 
thereby  avoiding  the  loss  of  magnetism  that  might  result  in  the 
permanent  magnet  by  the  action  of  heat.  Mr.  Tesla  also  provides 
for  lessening  the  volume  of  the  heat  or  for  intercepting  the  same 
during  that  portion  of  the  reciprocation  in  which  the  cooling 
action  takes  place. 

In  the  diagrams  are  shown  some  of  the  numerous  arrangements 
that  may  be  made  use  of  in  carrying  out  this  idea.  In  all 
of  these  figures  the  magnet-poles  are  marked  N  s,  the  armature 
A,  the  Bunsen  burner  or  other  source  of  heat  H,  the  axis  of  mo- 


FIG.  232. 


FIG.  231. 


FIG.  233. 


tion   M,  and  the  spring  or  the   equivalent   thereof — namely,  a 
weight — is  marked  w. 

In  Fig.  281  the  permanent  magnet  N  is  connected  with  a  frame, 
F,  supporting  the  axis  M,  from  which  the  arm  P  hangs,  and  at  the 
lower  end  of  which  the  armature  A  is  supported.  The  stops  2 
and  :-J  limit  the  extent  of  motion,  and  the  spring  w  tends  to  draw 
the  armature  A  away  from  the  magnet  N.  It  will  now  be  under- 
stood that  the  magnetism  of  >•  is  sufficient  to  overcome  the 
spring  w  and  draw  the  armature  A  toward  the  magnet  N.  The 
heat  acting  upon  the  armature  A  neutralizes  its  induced  magnet- 
ism sufficiently  for  the  spring  w  to  draw  the  armature  A  away 
from  the  magnet  M  and  also  from  the  heat  at  ir.  The  armature 
now  cools,  and  the  attraction  of  the  magnet  N  overcomes  the 
spring  w  and  draws  the  armature  A  back  again  above  the  burm-i- 


42(5 


INVENTIONS  OF  NIKOLA  TESLA. 


ii,  so  that  the  same  is  again  heated  and  the  operations  are  re- 
peated. The  reciprocating  movements  thus  obtained  are  em- 
ployed as  a  source  of  mechanical  power  in  any  desired  manner. 
Usually  a  connecting-rod  to  a  crank  upon  a  fly-wheel  shaft  would 
be  made  use  of,  as  indicated  in  Fig.  240. 

Fig.  232  represents  the  same  parts  as  before  described ;  but  an 


Fie.  234. 


FIG.  235. 


electro-magnet  is  illustrated  in  place  of  a  permanent  magnet. 
The  operations,  however,  are  the  same. 

In  Fig.  233  are  shown  the  same  parts  as  in  Figs.  231  and  232, 
but  they  are  differently  arranged.  The  armature  A,  instead  of 
swinging,  is  stationary  and  held  by  arm  p',  and  the  core  N  s  of 
the  electro-magnet  is  made  to  swing  within  the  helix  Q,  the 
core  being  suspended  by  the  arm  p  from  the  pivot  M.  A  shield, 
R,  is  connected  with  the  magnet-core  and  swings  with  it,  so 
that  after  the  heat  has  demagnetized  the  armature  A  to  such  an 
extent  that  the  spring  w  draws  the  core  N  s  away  from  the  arma- 
ture A,  the  shield  K  comes  between  the  flame  H  and  armature  A, 
thereby  intercepting  the  action  of  the  heat  and  allowing  the  ar- 
mature to  cool,  so  that  the  magnetism,  again  preponderating, 
causes  the  movement  of  the  core  N  s  toward  the  armature  A  and 
the  removal  of  the  shield  R  from  above  the  flame,  so  that  the  heat 
again  acts  to  lessen  or  neutralize  the  magnetism.  A  rotary  or 
other  movement  may  be  obtained  from  this  reciprocation. 

Fig.  234  corresponds  in  every  respect  with  Fig.  233,  except 
that  a  permanent  horseshoe-magnet,  N  s  is  represented  as  taking 
the  place  of  the  electro-magnet  in  Fig.  233. 

In  Fig.  235  is  shown  a  helix,  Q,  with  an  armature  adapted  to 
swing  toward  or  from  the  helix.  In  this  case  there  may  be  a  soft- 


THERMO-MAGNETI8M  AND  PJEO-MAGNETI8M. 


4-21 


iron  core  in  the  helix,  or  the  armature  may  assume  the  form  of  a 
solenoid  core,  there  being  no  permanent  core  within  the  helix. 

Fig.  23tf  is  an  end  view,  and  Fig.  237  a  plan  view,  illustrating 
the  method  as  applied  to  a  swinging  armature,  A,  and  a  stationary 
permanent  magnet,  N  s.  In  this  instance  Mr.  Tesla  applies  the 
heat  to  an  auxiliary  armature  or  keeper,  T,  which  is  adjacent  to 
and  preferably  in  direct  contact  with  the  magnet.  This  arma- 
ture T,  in  the  form  of  a  plate  of  sheet-iron,  extends  across  from 
one  pole  to  the  other  and  is  of  sufficient  section  to  practically 
form  a  keeper  for  the  magnet,  so  that  when  the  armature  T  is 
cool  nearly  all  the  lines  of  force  pass  over  the  same  and  very  little 
free  magnetism  is  exhibited.  Then  the  armature  A,  which  swings 
freely  on  the  pivots  M  in  front  of  the  poles  N  s,  is  very  little  at- 
tracted and  the  spring  w  pulls  the  same  way  from  the  poles  into 
the  position  indicated  in  the  diagram.  The  heat  is  directed  upon 
the  iron  plate  T  at  some  distance  from  the  magnet,  so  as  to  allow 
the  magnet  to  keep  comparatively  cool.  This  heat  is  applied  be- 
neath the  plate  by  means  of  the  burners  H,  and  there  is  a  con- 
nection from  the  armature  A  or  its  pivot  to  the  gas-cock  6,  or 
other  device  for  regulating  the  heat.  The  heat  acting  upon  the 
middle  portion  of  the  plate  T,  the  magnetic  conductivity  of  the 
heated  portion  is  diminished  or  destroyed,  and  a  great  number  of 
the  lines  of  force  are  deflected  over  the  armature  A,  which  is  now 


FIG.  287. 


FIG.  238. 


FIG. 


powerfully  attracted  and  drawn  into  line,  or  nearly  so,  with  the 
poles  N  s.  In  so  doing  the  cock  6  is  nearly  closed  and  the  plate 
T  cools,  the  lines  of  force  are  again  deflected  over  the  same,  the 
attraction  exerted  upon  the  armature  A  is  diminished,  and  the 
spring  w  pulls  the  same  away  from  the  magnet  into  the  position 
shown  by  full  lines,  and  the  operations  are  repeated. 


The  ar- 


438 


INVENTIONS  OF  NIKOLA  TE8LA 


rangement  shown  in  Fig.  236  has  the  advantages  that  the  mag- 
net and  armature  are  kept  cool  and  the  strength  of  the  per- 
manent magnet  is  better  preserved,  as  the  magnetic  circuit  is 
constantly  closed. 

In  the  plan  view,  Fig.  238,  is  shown  a  permanent  magnet  and 
keeper  plate,  T,  similar  to  those  in  Figs.  236  and  237,  with  the 
burners  H  for  the  gas  beneath  the  same ;  but  the  armature  is 
pivoted  at  one  end  to  one  pole  of  the  magnet  and  the  other  end 
swings  toward  and  from  the  other  pole  of  the  magnet.  The  spring 
w  acts  against  a  lever  arm  that  projects  from  the  armature,  and 
the  supply  of  heat  has  to  be  partly  cut  oif  by  a  connection  to  the 
swinging  armature,  so  as  to  lessen  the  heat  acting  upon  the  keeper 
plate  when  the  armature  A  has  been  attracted. 


0.;N 


FIG.  240. 


FIG.  241. 


Fig.  239  is  similar  to  Fig.  238,  except  that  the  keeper  T  is  not 
made  use  of  and  the  armature  itself  swings  into  and  out  of  the 
range  of  the  intense  action  of  the  heat  from  the  burner  H.  Fig. 
240  is  a  diagram  similar  to  Fig.  231,  except  that  in  place  of  using  a 
spring  and  stops,  the  armature  is  shown  as  connected  by  a  link, 
to  the  crank  of  a  fly-wheel,  so  that  the  fly-wheel  will  be  revolved 
as  rapidly  as  the  armature  can  be  heated  and  cooled  to  the 
necessary  extent.  A  spring  may  be  used  in  addition,  as  in  Fig. 
231.  In  Fig.  241  the  armatures  A  A  are  connected  by  a  link,  so 
that  one  will  be  heating  while  the  other  is  cooling,  and  the  attrac- 
tion exerted  to  move  the  cooled  armature  is  availed  of  to  draw 
away  the  heated  armature  instead  of  using  a  spring. 


THEKMO-MAGNETISM  AND  PT110-MAGNETISM.  429 

Mr.  Tesla  has  also  devoted  his  attention  to  the  development  of 
a  pyromagnetic  generator  of  electricity1  based  upon  the  following 
laws :  First,  that  electricity  or  electrical  energy  is  developed  in 
any  conducting  body  by  subjecting  such  body  to  a  varying  mag- 
netic influence  ;  and  second,  that  the  magnetic  properties  of  iron 
or  other  magnetic  substance  may  be  partially  or  entirely  destroyed- 
or  caused  to  disappear  by  raising  it  to  a  certain  temperature,  but 
restored  and  caused  to  reappear  by  again  lowering  its  tempera- 
ture to  a  certain  degree.  These  laws  may  be  applied  in  the  pro- 
duction of  electrical  currents  in  many  ways,  the  principle  of 
which  is  in  all  cases  the  same,  viz.,  to  subject  a  conductor  to  a 
varying  magnetic  influence,  producing  such  variations  by  the  ap- 
plication of  heat,  or,  more  strictly  speaking,  by  the  application  or 
action  of  a  varying  temperature  upon  the  source  of  the  magnet- 
ism. This  principle  of  operation  may  be  illustrated  by  a  simple 
experiment :  Place  end  to  end,  and  preferably  in  actual  contact, 
a  permanently  magnetized  steel  bar  and  a  strip  or  bar  of  soft  iron. 
Around  the  end  of  the  iron  bar  or  plate  wind  a  coil  of  insulated  wire. 
Then  apply  to  the  iron  between  the  coil  and  the  steel  bar  a  flame 
or  other  source  of  heat  which  will  be  capable  of  raising  that  por- 
tion of  the  iron  to  an  orange  red,  or  a  temperature  of  about  600° 
centigrade.  "When  this  condition  is  reached,  the  iron  somewhat 
suddenly  loses  its  magnetic  properties,  if  it  be  very  thin,  and  the 
same  effect  is  produced  as  though  the  iron  had  been  moved  away 
from  the  magnet  or  the  heated  section  had  been  removed.  This 
change  of  position,  however,  is  accompanied  by  a  shifting  of  the 
magnetic  lines,  or,  in  other  words,  by  a  variation  in  the  magnetic 
influence  to  which  the  coil  is  exposed,  and  a  current  in  the  coil 
is  the  result.  Then  remove  the  flame  or  in  any  other  way  reduce 
the  temperature  of  the  iron.  The  lowering  of  its  temperature  is 
accompanied  by  a  return  of  its  magnetic  properties,  and  another 
change  of  magnetic  conditions  occurs,  accompanied  by  a  current 
in  an  opposite  direction  in  the  coil.  The  same  operation  may  be 

1.  The  chief  point  to  be  noted  is  that  Mr.  Tesla  attacked  this  problem  in  a 
way  which  was,  from  the  standpoint  of  theory,  and  that  of  an  engineer,  far 
better  than  that  from  which  some  earlier  trials  in  this  direction  started.  The 
enlargement  of  these  ideas  will  be  found  in  Mr.  Tesla's  work  on  the  pyromag- 
netic generator,  treated  in  this  chapter.  The  chief  effort  of  the  inventor  was 
to  economize  the  heat,  which  was  accomplished  by  inclosing  the  iron  in  a  source 
of  heat  well  insulated,  and  by  cooling  the  iron  by  means  of  steam,  utilizing  the 
steam  over  again.  The  construction  also  permits  of  more  rapid  magnetic 
changes  per  unit  of  time,  meaning  larger  output. 


430 


INVENTIONS  OF  NIKOLA  TESLA. 


repeated  indefinitely,  the  effect  upon  the  coil  being  similar  to 
that  which  would  follow  from  moving  the  magnetized  bar  to  and 
from  the  end  of  the  iron  bar  or  plate. 

The  device  illustrated  below  is  a  means  of  obtaining  this 
result,  the  features  of  novelty  in  the  invention  being,  first,  the 
employment  of  an  artificial  cooling  device,  and,  second,  inclosing 
the  source  of  heat  and  that  portion  of  the  magnetic  circuit  ex- 
posed to  the  heat  and  artificially  cooling  the  heated  part. 

These  improvements  are  applicable  generally  to  the  generators 
constructed  on  the  plan  above  described — that  is  to  say,  we  may 
use  an  artificial  cooling  device  in  conjunction  with  a  variable  or 
varied  or  uniform  source  of  heat. 

Fig.  242  is  a  central  vertical  longitudinal  section  of  the  com- 


FIG.  242. 


FIG.  243. 


plete  apparatus  and  Fig.  243  is  a  cross-section  of  the  magnetic 
armature-core  of  the  generator. 

Let  A  represent  a  magnetized  core  or  permanent  magnet  the 
poles  of  which  are  bridged  by  an  armature-core  composed  of  a 
casing  or  shell  B  inclosing  a  number  of  hollow  iron  tubes  c. 
Around  this  core  are  wound  the  conductors  E  E',  to  form  the 
coils  in  which  the  currents  are  developed.  In  the  circuits  of 
these  coils  are  current-consuming  devices,  as  F  F'. 

n  is  a  furnace  or  closed  fire-box,  through  which  the  central 
portion  of  the  core  B  extends.  Above  the  fire  is  a  boiler  K,  con- 
taining water.  The  flue  L  from  the  fire-box  may  extend  up 
through  the  boiler. 

G  is  a  water-supply  pipe,  and  H  is  the  steam-exhaust  pipe, 
which  communicates  with  all  the  tubes  c  in  the  armature  B,  so 
that  steam  escaping  from  the  boiler  will  pass  through  the  tubes. 


THKRMO-MA  ONETISM  AND  P7RO-MA  QNETISM.  431 

In  tlie  steam-exhaust  pipe  H  is  a  valve  v,  to  which  is  connected 
the  lever  i,  by  the  movement  of  which  the  valve  is  opened 
or  closed.  In  such  a  case  as  this  the  heat  of  the  fire  may  he 
utilized  for  other  purposes  after  as  much  of  it  as  may  be  needed 
has  been  applied  to  heating  the  core  u.  There  are  special  ad- 
vantages in  the  employment  of  a  cooling  device,  in  that  the 
metal  of  the  core  B  is  not  so  quickly  oxidized.  Moreover,  the 
difference  between  the  temperature  of  the  applied  heat  and  of 
the  steam,  air,  or  whatever  gas  or  fluid  be  applied  as  the  cooling 
medium,  may  be  increased  or  decreased  at  will,  whereby  the 
rapidity  of  the  magnetic  changes  or  fluctuations  may  be  regulated. 


CHAPTEK  XXXVII. 
ANTI-SPAKKING  DYNAMO  BRUSH  AND  COMMUTATOR. 

IN  direct  current  dynamos  of  great  electromotive  force — such, 
for  instance,  as  those  used  for  arc  lighting — when  one  commuta- 
tor bar  or  plate  comes  out  of  contact  with  the  collecting-brush  a 
spark  is  apt  to  appear  on  the  commutator.  This  spark  may  be 
due  to  the  break  of  the  complete  circuit,  or  to  a  shunt  of  low 
resistance  formed  by  the  brush  between  two  or  more  commuta- 
tor-bars. In  the  lirst  case  the  spark  is  more  apparent,  as  there  is 
at  the  moment  when  the  circuit  is  broken  a  discharge  of  the 
magnets  through  the  field  helices,  producing  a  great  spark  or 
Hash  which  causes  an  unsteady  current,  rapid  wear  of  the  com- 
mutator bars  and  brushes,  and  waste  of  power.  The  sparking 
may  be  reduced  by  various  devices,  such  as  providing  a  path  for 
the  current  at  the  moment  when  the  commutator  segment  or  bar 
leaves  the  brush,  by  short-circuiting  the  field-helices,  by  increas- 
ing the  number  of  the  commutator-bars,  or  by  other  similar 
means  ;  but  all  these  devices  are  expensive  or  not  fully  available, 
and  seldom  attain  the  object  desired. 

To  prevent  this  sparking  in  a  simple  manner,  Mr.  Tesla  some 
years  ago  employed  with  the  commutator-bars  and  intervening 
insulating  material,  mica,  asbestos  paper  or  other  insulating  and 
incombustible  material,  arranged  to  bear  on  the  surface  of  the 
commutator,  near  to  and  behind  the  brush. 

In  the  drawings,  Fig.  244  is  a  section  of  a  commutator  with 
an  asbestos  insulating  device ;  and  Fig.  245  is  a  similar  view,  re- 
presenting two  plates  of  mica  upon  the  back  of  the  brush. 

In  Fig.  244,  c  represents  the  commutator  and  intervening 
insulating  material ;  B  B,  the  brushes,  d  d  are  sheets  of  asbestos 
paper  or  other  suitable  non-conducting  material.  //  are  springs, 
the  pressure  of  which  may  be  adjusted  by  means  of  the  screws 

V  9- 

In  Fig.  245  a  simple  arrangement  is  shown  with  two  plates  of 
mica  or  other  material.  It  will  be  seen  that  whenever  one  com- 


ANTL SPARKING  BRUSHES  AND  COMMUTATORS.  433 

imitator  segment  passes  out  of  contact  with  the  brush,  the  forma- 
tion of  the  arc  will  be  prevented  by  the  intervening  insulating 
material  coming  in  contact  with  the  insulating  material  on  the 
brush. 

Asbestos  paper  or  cloth  impregnated  with  zinc-oxide,  mag- 
nesia, zirconia,  or  other  suitable  material,  may  be  used,  as  the 


FIG.  244. 


FIG.  245. 


paper  and  cloth  are  soft,  and  serve  at  the  same  time  to  wipe  and 
polish  the  commutator ;  but  mica  or  any  other  suitable  material 
can  be  employed,  provided  the  material  be  an  insulator  or  a  bad 
conductor  of  electricity. 

A  few  years  later  Mr.  Tesla  turned  his  attention  again  to  the 
same  subject,  as,  perhaps,  was  very  natural  in  view  of  the  fact 
that  the  commutator  had  always  been  prominent  in  his  thoughts, 
and  that  so  much  of  his  work  was  even  aimed  at  dispensing  with 
it  entirely  as  an  objectionable  and  unnecessary  part  of  dynamos 
and  motors.  In  these  later  efforts  to  remedy  commutator  troubles, 
Mr.  Tesla  constructs  a  commutator  and  the  collectors  therefor  in 
two  parts  mutually  adapted  to  one  another,  and,  so  far  as  the  es- 
sential features  are  concerned,  alike  in  mechanical  structure.  Se- 
lecting as  an  illustration  a  commutator  of  two  segments  adapted 
for  use  with  an  armature  the  coils  or  coil  of  which  have  but  two 
free  ends,  connected  respectively  to  the  segments,  the  bearing- 
surface  is  the  face  of  a  disc,  and  is  formed  of  two  metallic  quad- 
rant segments  and  two  insulating  segments  of  the  same  dimensions, 
and  the  face  of  the  disc  is  smoothed  off,  so  that  the  metal 
and  insulating  segments  are  flush.  The  part  which  takes  the 
place  of  the  usual  brushes,  or  the  "  collector,"  is  a  disc  of  the 
same  character  as  the  commutator  and  has  a  surface  similarly 
formed  with  two  insulating  and  two  metallic  segments.  These 
two  parts  are  mounted  with  their  faces  in  contact  and  in  such 
manner  that  the  rotation  of  the  armature  causes  the  commutator 
to  turn  upon  the  collector,  whereby  the  currents  induced  in  the 


434 


INVENTIONS  OF  NIKOLA  TESLA. 


coils  are  taken  off  by  the  collector  segments  and  thence  conveyed 
off  by  suitable  conductors  leading  from  the  collector  segments. 
This  is  the  general  plan  of  the  construction  adopted.  Aside  from 
certain  adjuncts,  the  nature  and  functions  of  which  are  set  forth 
later,  this  means  of  commutation  will  be  seen  to  possess  many  im- 
portant advantages.  In  the  first  place  the  short-circuiting  and  the 
breaking  of  the  armature  coil  connected  to  the  commutator-seg- 
ments occur  at  the  same  instant,  and  from  the  nature  of  the  con- 
struction this  will  be  done  with  the  greatest  precision  ;  secondly,  the 
duration  of  both  the  break  and  of  the  short  circuit  will  be  reduced 
to  a  minimum.  The  first  results  in  a  reduction  which  amounts 
practically  to  a  suppression  of  the  spark,  since  the  break  and 
the  short  circuit  produce  opposite  effects  in  the  armature-coil. 
The  second  has  the  effect  of  diminishing  the  destructive  effect 
of  a  spark,  since  this  would  be  in  a  measure  proportional  to  the 
duration  of  the  spark;  while  lessening  the  duration  of  the  short 
circuit  obviously  increases  the  efficiency  of  the  machine. 


FIG.  246. 


FIG.  247. 


The  mechanical  advantages  will  be  better  understood  by  re- 
ferring to  the  accompanying  diagrams,  in  which  Fig.  246  is  a 
central  longitudinal  section  of  the  end  of  a  shaft  with  the  im- 
proved commutator  carried  thereon.  Fig.  247  is  a  view  of  the 
inner  or  bearing  face  of  the  collector.  Fig.  248  is  an  end  view 
from  the  armature  side  of  a  modified  form  of  commutator.  Figs. 


ANTI-SPARKING  BRUSHES  AND  COMMUTATORS.  435 

249  and  250  are  views  of  details  of  Fig.  248.  Fig.  251  is  a  longi- 
tudinal central  section  of  another  modification,  and  Fig.  252  is  a 
sectional  view  of  the  same.  A  is  the  end  of  the  armature-shaft 
of  a  dynamo-electric  machine  or  motor.  A'  is  a  sleeve  of  insu- 
lating material  around  the  shaft,  secured  in  place  by  a  screw  a'. 


FIG.  248  FIG.  249.  FIG.  250. 

The  commutator  proper  is  in  the  form  of  a  disc  which  is  made 
up  of  four  segments  n  D'  G  G',  similar  to  those  shown  in  Fig.  248. 
Two  of  these  segments,  as  D  D',  are  of  metal  and  are  in  electrical 
connection  with  the  ends  of  the  coils  on  the  armature.  The 
other  two  segments  are  of  insulating  material.  The  segments  are 
held  in  place  by  a  band,  B,  of  insulating  material.  The  disc  is 
held  in  place  by  friction  or  by  screws,  y'  g' ,  Fig.  248,  which 
secure  the  disc  firmly  to  the  sleeve  A'. 

The  collector  is  made  in  the  same  form  as  the  commutator.  It 
is  composed  of  the  two  metallic  segments  E  E'  and  the  two  insu- 
lating segments  r  F',  bound  together  by  a  band,  c.  The  metallic 
segments  E  E'  are  of  the  same  or  practically  the  same  width  or 
extent  as  the  insulating  segments  or  spaces  of  the  commutator. 
The  collector  is  secured  to  a  sleeve,  B',  by  screws  g  g,  and  the  sleeve 
is  arranged  to  turn  freely  on  the  shaft  A.  The  end  of  the  sleeve 
B'  is  closed  by  a  plate,  /,  upon  which  presses  a  pivot-pointed 
screw,  /i,  adjustable  in  a  spring,  H,  which  acts  to  maintain  the 
collector  in  close  contact  with  the  commutator  and  to  compensate 
for  the  play  of  the  shaft.  The  collector  is  so  fixed  that  it  cannot 
turn  with  the  shaft.  For  example,  the  diagram  shows  a  slotted 
plate,  K,  which  is  designed  to  be  attached  to  a  stationary  support, 
and  an  arm  extending  from  the  collector  and  carrying  a  clamping 
screw,  L,  by  which  the  collector  may  be  adjusted  and  set  to  the 
desired  position. 

Mr.  Tesla  prefers  the  form  shown  in  Figs.  246  and  247  to  fit 


436 


INVENTIONS  OF  NIKOLA  TESLA. 


the  insulating  segments  of  both  commutator  and  collector  loosely 
and  to  provide  some  means — as,  for  example,  light  springs,  e  e, 
secured  to  the  bands  A'  B',  respectively,  and  bearing  against  the 
segments — to  exert  a  light  pressure  upon  them  and  keep  them  in 
close  contact  and  to  compensate  for  wear.  The  metal  segments 
of  the  commutator  may  be  moved  forward  by  loosening  the 
screw  a'. 

The  line  wires  are  fed  from  the  metal  segments  of  the  collector, 
being  secured  thereto  in  any  convenient  manner,  the  plan  of  con- 
nections being  shown  as  applied  to  a  modified  form  of  the  com- 
mutator in  Fig.  251.  The  commutator  and  the  collector  in  thus 
presenting  two  flat  and  smooth  bearing  surfaces  prevent  most  ef- 
fectually by  mechanical  action  the  occurrence  of  sparks. 

The  insulating  segments  are  made  of  some  hard  material  capa- 
ble of  being  polished  and  formed  with  sharp  edges.  Such  mater- 
ials as  glass,  marble,  or  soapstone  may  be  advantageously  used. 
The  metal  segments  are  preferably  of  copper  or  brass ;  but  they 
may  have  a  facing  or  edge  of  durable  material — such  as  platinum 
or  the  like — where  the  sparks  are  liable  to  occur. 

In  Fig.  248  a  somewhat  modified  form  of  the  invention  is 
shown,  a  form  designed  to  facilitate  the  construction  and  replac- 


FIG.  251. 


FIG.  252. 


ing  of  the  parts.  In  this  modification  the  commutator  and  col- 
lector are  made  in  substantially  the  same  manner  as  previously 
described,  except  that  the  bands  B  o  are  omitted.  The  four  seg- 
ments of  each  part,  however,  are  secured  to  their  respective  sleeves 
by  screws  g'  g' ,  and  one  edge  of  each  segment  is  cut  away,  so  that 
small  plates  a  b  may  be  slipped  into  the  spaces  thus  formed.  Of 


ANTI-SPARKINO  BRUSHKS  AND  COMMUTATORS.  437 

these  plates  a  a  are  of  metal,  and  are  in  contact  with  the  metal  seg- 
ments D  D',  respectively.  The  other  two,  b  &,  are  of  glass  or  mar- 
ble, and  they  are  all  better  square,  as  shown  in  Figs.  249  and  250, 
so  that  they  may  be  turned  to  present  new  edges  should  any  edge 
become  worn  by  use.  Light  springs  d  bear  upon  these  plates 
and  press  those  in  the  commutator  toward  those  in  the  collector, 
and  insulating  strips  c  c  are  secured  to  the  periphery  of  the  discs 
to  prevent  the  blocks  from  being  thrown  out  by  centrifugal  action. 
These  plates  are,  of  course,  useful  at  those  edges  of  the  segments 
only  where  sparks  are  liable  to  occur,  and,  as  they  are  easily  re- 
placed, they  are  of  great  advantage.  It  is  considered  best  to  coat 
them  with  platinum  or  silver. 

In  Figs.  251  and  252  is  shown  a  construction  where,  instead  of 
solid  segments,  a  fluid  is  employed.  In  this  case  the  commuta- 
tor and  collector  are  made  of  two  insulating  discs,  s  T,  and  in 
lieu  of  the  metal  segments  a  space  is  cut  out  of  each  part,  as  at 
K  K',  corresponding  in  shape  and  size  to  a  metal  segment.  The 
two  parts  are  iitted  smoothly  and  the  collector  T  held  by  the 
screw  h  and  spring  n  against  the  commutator  s.  As  in  the  other 
cases,  the  commutator  revolves  while  the  collector  remains  sta- 
tionary. The  ends  of  the  coils  are  connected  to  binding-posts 
$  -v,  which  are  in  electrical  connection  with  metal  plates  t  2  within 
the  recesses  in  the  two  parts  s  T.  These  chambers  or  recesses 
are  filled  with  mercury,  and  in  the  collector  part  are  tubes  w  w, 
with  screws  w  w,  carrying  springs  x  and  pistons  x',  which  com- 
pensate for  the  expansion  and  contraction  of  the  mercury  under 
varying  temperatures,  but  which  are  sufficiently  strong  not  to 
yield  to  the  pressure  of  the  fluid  due  to  centrifugal  action,  and 
which  serve  as  binding-posts. 

In  all  the  above  cases  the  commutators  are  adapted  fora  single 
coil,  and  the  device  is  particularly  suited  to  such  purposes.  The 
number  of  segments  may  be  increased,  however,  or  more  than 
one  commutator  used  with  a  single  armature.  Although  the 
bearing-surfaces  are  shown  as  planes  at  right  angles  to  the  shaft 
or  axis,  it  is  evident  that  in  this  particular  the  construction  'may 
be  very  greatly  modified. 


CHAPTER   XXXVIII. 
AUXILIARY  BKUSH  REGULATION  OF  DIRECT  CURRENT  DYNAMOS. 

AN  interesting  method  devised  by  Mr.  Tesla  for  the  regula- 
tion of  direct  current  dynamos,  is  that  which  lias  come  to  be 
known  as  the  "third  brush"  method.  In  machines  of  this  type, 
devised  by  him  as  far  back  as  1885,  he  makes  use  of  two  main 
brushes  to  which  the  ends  of  the  field  magnet  coils  are  connected, 
an  auxiliary  brush,  and  a  branch  or  shunt  connection  from  an  in- 
termediate point  of  the  iield  wire  to  the  auxiliary  brush.1 

The  relative  positions  of  the  respective  brushes  are  varied, 
either  automatically  or  by  hand,  so  that  the  shunt  becomes  in- 
operative when  the  auxiliary  brash  has  a  certain  position  upon 
the  commutator ;  but  when  the  auxiliary  brush  is  moved  in  its 
relation  to  the  main  brushes,  or  the  latter  are  moved  in  their 
relation  to  the  auxiliary  brush,  the  electric  condition  is  disturbed 
and  more  or  less  of  the  current  through  the  field-helices  is 
diverted  through  the  shunt  or  a  current  is  passed  over  the  shunt 
to  the  field-helices.  By  varying  the  relative  position  upon  the 
commutator  of  the  respective  brushes  automatically  in  propor- 
tion to  the  varying  electrical  conditions  of  the  working-circuit, 
the  current  developed  can  be  regulated  in  proportion  to  the  de- 
mands in  the  working-circuit. 

Fig.  253  is  a  diagram  illustrating  the  invention,  showing  one 
core  of  the  field-magnets  with  one  helix  wound  in  the  same  direc- 
tion throughout.  Figs.  254  and  255  are  diagrams  showing  one 
core  of  the  field-magnets  with  a  portion  of  the  helices  wound  in 
opposite  directions.  Figs.  256  and  257  are  diagrams  illustrating 

1.  The  compiler  has  learned  partially  from  statements  made  on  several 
occasions  in  journals  and  partially  by  personal  inquiry  of  Mr.  Tesla,  that  a 
great  deal  of  work  in  this  interesting  line  is  unpublished.  In  these  inventions 
as  will  be  seen,  the  brushes  are  automatically  shifted,  but  in  the  broad  method 
barely  suggested  here  the  regulation  is  effected  without  any  change  in  the 
position  of  the  brushes.  This  auxiliary  brush  invention,  it  will  be  remem- 
bered, was  very  much  discussed  a  few  years  ago,  and  it  may  be  of  interest  that 
this  work  of  Mr.  Tesla,  then  unknown  in  this  field,  is  now  brought  to  light 


A  UXILIA  R  Y  BR  U8U  REG  ULA  TION.  439 

the  electric  devices  that  may  be  employed  for  automatically 
adjusting  the  brushes,  and  Fig.  258  is  a  diagram  illustrating  the 
positions  of  the  brushes  when  the  machine  is  being  energized  at 
the  start. 

a  and  5  are  the  positive  and  negative  brushes  of  the  main  or 
working-circuit,  and  c  the  auxiliary  brush.  The  working-circuit 
i)  extends  from  the  brushes  a  and  b,  as  usual,  and  contains  elec- 
tric lamps  or  other  devices,  D',  either  in  series  or  in  multiple 
arc. 

M  M'  represent  the  field-helices,  the  ends  of  which  are  con- 
nected to  the  main  brushes  a  and  5.  The  branch  or  shunt  wire 
c'  extends  from  the  auxiliary  brush  c  to  the  circuit  of  the  field- 
helices,  and  is  connected  to  the  same  at  an  intermediate  point,  v. 

H  represents  the  commutator,  with  the  plates  of  ordinary  con- 


FIG.  253. 

struction.  When  the  auxiliary  brush  c  occupies  such  a  position 
upon  the  commutator  that  the  electro-motive  force  between  the 
brushes  a  and  c  is  to  the  electro-motive  force  between  the  brushes 
c  and  b  as  the  resistance  of  the  circuit  a  M  c'  c  A  is  to  the  resistance 
of  the  circuit  b  M'  c'  c  B,  the  potentials  of  the  points  x  and  Y  will 
be  equal,  and  no  current  will  flow  over  the  auxiliary  brush ;  but 
when  the  brush  c  occupies  a  different  position  the  potentials  of 
the  points  x  and  Y  will  be  different,  and  a  current  will  flow  over 
the  auxiliary  brush  to  and  from  the  commutator,  according  to  the 
relative  position  of  the  brushes.  If,  for  instance,  the  commu- 
tator-space between  the  brushes  a  and  c,  when  the  latter  is  at  the 
neutral  point,  is  diminished,  a  current  will  flow  from  the  point  Y 
over  the  shunt  c  to  the  brush  J,  thus  strengthening  the  current 
in  the  part  M',  and  partly  neutralizing  the  current  in  part  M  ;  but 
if  the  space  between  the  brushes  a  and  c  is  increased,  the  cur- 


440 


INVENTIONS  OF  NIKOLA  TESLA. 


rent  will  flow  over  the  auxiliary  brush  in  an  opposite  direction, 
and  the  current  in  M  will  be  strengthened,  and  in  M'  partly  neu- 
tralized. 

By  combining  with  the  brushes  a,  I,  and  c  any  usual  automatic 
regulating  mechanism,  the  current  developed  can  be  regulated  in 
proportion  to  the  demands  in  the  working  circuit.  The  parts  M 


FIG.  254. 

and  M'  of  the  Held  wire  may  be  wound  in  the  same  direction. 
In  this  case  they  are  arranged  as  shown  in  Fig.  253  ;  or  the  part 
M  may  be  wound  in  the  opposite  direction,  as  shown  in  Figs. 

254  and  255. 

It  will  be  apparent  that  the  respective  cores  of  the  tield-rnag- 
nets  are  subjected  to  neutralizing  or  intensifying  effects  of  the 
current  in  the  shunt  through  c',  and  the  magnetism  of  the  cores 
will  be  partially  neutralized,  or  the  points  of  greatest  magnetism 
shifted,  so  that  it  will  be  more  or  less  remote  from  or  approach- 
ing to  the  armature,  and  hence  the  aggregate  energizing  actions 
of  the  field  magnets  on  the  armature  will  be  correspondingly 
varied. 

In  the  form  indicated  in  Fig.  253  the  regulation  is  effected  by 
shifting  the  point  of  greatest  magnetism,  and  in  Figs.  254  and 

255  the  same  effect  is  produced  by  the  action  of  the  current  in 
the  shunt  passing  through  the   neutralizing  helix. 

The  relative  positions  of  the  respective  brushes  may  be  varied 
by  moving  the  auxiliary  brush,  or  the  brush  c  may  remain  station- 
ary and  the  core  P  be  connected  to  the  main-brush  holder  A, 
so  as  to  adjust  the  brushes  a  b  in  their  relation  to  the  brush  c. 
If,  however,  an  adjustment  is  applied  to  all  the  brushes,  as  seen 
in  Fig.  257,  the  solenoid  should  be  connected  to  both  a  and  c,  so 
as  to  move  them  toward  or  away  from  each  other. 

There  are  several  known  devices  for  giving  motion  in  propor- 


A  UXfLIARY  BRUSH  It  KG  ULAT1ON. 


441 


tion  to  an  electric  current.  In  Figs.  25»i  and  257  the  moving 
cores  are  shown  as  convenient  devices  for  obtaining  the  required 
extent  of  motion  with  very  slight  changes  in  the  current  passing 
through  the  helices.  It  is  understood  that  the  adjustment  of 
the  main  brushes  causes  variations  in  the  strength  of  the  current 
independently  of  the  relative  position  of  those  brushes  to  the 
auxiliary  brush.  In  all  cases  the  adjustment  should  be  such  that 
no  current  flows  over  the  auxiliary  brush  when  the  dynamo  is 
running  with  its  normal  load. 

In  Figs.  256  and  25 7  A  A  indicate  the  main-brush  holder, 
carrying  the  main  brushes,  and  c  the  auxiliary-brush  holder, 
carrying  the  auxiliary  brush.  These  brush-holders  are  movable 
in  arcs  concentric  with  the  centre  of  the  commutator-shaft.  An 
iron  piston,  p,  of  the  solenoid  s,  Fig.  25(5,  is  attached  to  the  aux- 
iliary-brush holder  c;.  The  adjustment  is  effected  by  means  of  a 
spring  and  screw  or  tightener. 

In  Fig.  257  instead  of  a  solenoid,  an  iron  tube  inclosing  a  coil 
is  shown.  The  piston  of  the  coil  is  attached  to  both  brush- 
holders  A  A  and  c.  When  the  brushes  are  moved  directly  by 
electrical  devices,  as  shown  in  Figs.  25(5  and  257,  these  are  so 
constructed  that  the  force  exerted  for  adjusting  is  practically 
uniform  through  the  whole  length  of  motion. 

It  is  true  that  auxiliary  brushes  have  been  used  in  connection 
with  the  helices  of  the  field-wire;  but  in  these  instances  the 


FIG.  255. 

helices  receive  the  entire  current  through  the  auxiliary  brush  or 
brushes,  and  these  brushes  could  not  be  taken  off  without  break- 
ing the  circuit  through  the  field.  These  brushes  cause,  move- 
over,  heavy  sparking  at  the  commutator.  In  the  present 
case  the  auxiliary  brush  causes  very  little  or  no  sparking,  and 
can  be  taken  off  without  breaking  the  circuit  through  the  field- 


442 


INVENTIONS  OF  NIKOLA  TESLA. 


helices.  The  arrangement  lias,  besides,  the  ad  vantage  of  facilitating 
the  self-excitation  of  the  machine  in  all  cases  where  the  resis- 
tance of  the  field- wire  is  very  great  comparatively  to  the  resis-' 
tance  of  the  main  circuit  at  the  start — for  instance,  on  arc-light 


FIG.  256. 


machines.  In  this  case  the  auxiliary  brush  e  is  placed  near  to,  or 
better  still  in  contact  with,  the  brush  £,  as  shown  in  Fig.  258. 
In  this  manner  the  part  M'  is  completely  cut  out,  and  as  the  part 
M  has  a  considerably  smaller  resistance  than  the  whole  length  of 
the  field-wire  the  machine  excites  itself,  whereupon  the  auxiliary 
brush  is  shifted  automatically  to  its  normal  position. 

In  a  further  method  devised  by  Mr.  Tesla,  one  or  more  auxili- 
ary brushes  are  employed,  by  means  of  which  a  portion  or  the 
whole  of  the  field  coils  is  shunted.  According  to  the  relative  po- 
sition upon  the  commutator  of  the  respective  brushes  more  or 
less  current  is  caused  to  pass  through  the  helices  of  the  field,  and 
the  current  developed  by  the  machine  can  be  varied  at  will  by 
varying  the  relative  positions  of  the  brushes. 

In  Fig.  259,  a  and  1)  are  the  positive  and  negative  brushes  of 
the  main  circuit,  and  c  an  auxiliary  brush.  The  main  circuit  D 


FIG.  258. 

extends  from  the  brushes  a  and  b,  as  usual,  and  contains  the 
helices  M  of  the  field  wire  and  the  electric  lamps  or  other  work- 
ing devices.  The  auxiliary  brush  c  is  connected  to  the  point  x 
of  the  main  circuit  by  means  of  the  wire  c' .  H  is  a  commutator 


A  UXILIAR  T  Bit  USE  REG  ULA  TION.  443 

of  ordinary  construction.  It  will  have  been  seen  from  what  was 
said  already  that  when  the  electro-motive  force  between  the  brushes 
a  and  c  is  to  the  electromotive  force  between  the  brushes  c 
and  b  as  the  resistance  of  the  circuit  a  M  c'  c  A  is  to  the  resistance 
of  the  circuit  b  c  B  c  c'  D,  the  potentials  of  the  points  a?  and  y 
will  be  equal,  and  no  current  will  pass  over  the  auxiliary  brush 
c;  but  if  that  brush  occupies  a  different  position  relatnely  to  the 
main  brushes  the  electric  condition  is  disturbed,  and  current 
will  flow  either  from  y  to  x  or  from  a?  to  y,  according  to  the  rela- 
tive position  of  the  brushes.  In  the  first  case  the  current  through 
the  field-helices  will  be  partly  neutralized  and  the  magnetism  of 
the  field  magnets  will  be  diminished.  In  the  second  case  the 
current  will  be  increased  and  the  magnets  gain  strength.  By 
combining  with  the  brushes  a  1)  c  any  automatic  regulating 
mechanism,  the  current  developed  can  be  regulated  automatically 
in  proportion  to  the  demands  of  the  working  circuit. 

In  Figs.  264  and  265  some  of  the  automatic  means  are  repre- 
sented that  may  be  used  for  moving  the  brushes.  The  core  P, 
Fig.  264,  of  the  solenoid-helix  s  is  connected  with  the  brush  c  to 
move  the  same,  and  in  Fig.  265  the  core  P  is  shown  as  within  the 
helix  s,  and  connected  with  brushes  a  and  c,  so  as  to  move  the 
same  toward  or  from  each  other,  according  to  the  strength  of  the 
current  in  the  helix,  the  helix  being  within  an  iron  tube,  s',  that 
becomes  magnetized  and  increases  the  action  of  the  solenoid. 

In  practice  it  is  sufficient  to  move  only  the  auxiliary  brush,  as 
shown  in  Fig.  264,  as  the  regulation  is  very  sensitive  to  the 
slightest  changes ;  but  the  relative  position  of  the  auxiliary  brush 
to  the  main  brushes  may  be  varied  by  moving  the  main  brushes, 
or  both  main  and  auxiliary  brushes  may  be  moved,  as  illustrated 
in  Fig.  265.  In  the  latter  two  cases,  it  will  be  understood,  the 
motion  of  the  main  brushes  relatively  to  the  neutral  line  of  the 
machine  causes  variations  in  the  strength  of  the  current  inde- 
pendently of  their  relative  position  to  the  auxiliary  brush.  In 
all  cases  the  adjustment  may  be  such  that  when  the  machine  is 
running  with  the  ordinary  load,  no  current  fiows  over  the  auxil- 
iary brush. 

The  field  helices  may  be  connected,  as  shown  in  Fig.  25!>,  or  a 
part  of  the  field  helices  may  be  in  the  outgoing  and  the  other  part 
in  the  return  circuit,  and  two  auxiliary  brushes  may  be  employed 
as  shown  in  Figs.  261  and  262.  Instead  of  shunting  the  whole 
of  the  field  helices,  a  portion  only  of  such  helices  maybe  shunted, 
as  shown  in  Figs.  260  and  262. 


444 


INVENTIONS  OF  NIKOLA  TESLA. 


The  arrangement  shown  in  Fig.  '2ti*2  is  advantageous,  as  it  dim- 
inishes the  sparking  upon  the  commutator,  the  main  circuit  being 
closed  through  the  auxiliary  brushes  at  the  moment  of  the  break 
of  the  circuit  at  the  main  brushes. 


FIG.  259. 


FIG.  261. 


FIG.  262. 


FIG.  263. 


The  field  helices  may  be  wound  in  the  same  direction,  or  a  part 
may  be  wound  in  opposite  directions. 

The  connection  between  the  helices  and  the  auxiliary  brush  or 
brushes  may  be  made  by  a  wire  of  small  resistance,  or  a  resistance 
may  be  interposed  (R,  Fig.  263,)  between  the  point  ./•  and  the 


A UXILIARY  BRUSH  REG ULATWN.  44~> 

auxiliary  brush  or  brushes  to  divide  the  sensitiveness  when  the 
'brushes  are  adjusted. 

The  accompanying  sketches  also  illustrate  improvements  made 
by  Mr.  Tesla  in  the  mechanical  devices  used  to  effect  the  shift- 
ing of  the  brushes,  in  the  use  of  an  auxiliary  brush.  Fig.  266  is 
an  elevation  of  the  regulator  with  the  frame  partly  in  section  ; 
and  Fig.  267  is  a  section  at  the  line  a?  a?,  Fig.  266.  c  is  the  com- 
mutator; B  and  B',  the  brush-holders,  B  carrying  the  main 
brushes  a  a' ,  and  B'  the  auxiliary  or  shunt  brushes  b  b.  The 
axis  of  the  brush-holder  B  is  supported  by  two  pivot-screws,  JP />. 
The  other  brush-holder,  B',  has  a  sleeve,  d,  and  is  movable 
around  the  axis  of  the  brush-holder  B.  In  this  way  both  brush- 
holders  can  turn  very  freely,  the  friction  of  the  parts  being 
reduced  to  a  minimum.  Over  the  brush-holders  is  mounted  the 
solenoid  s,  which  rests  upon  a  forked  column,  c.  This  column 


FIG.  264.  Fro.  265. 

also  affords  a  support  for  the  pivots p p,  and  is  fastened  upon  a 
solid  bracket  or  projection,  p,  which  extends  from  the  base  of 
the  machine,  and  is  cast  in  one  piece  with  the  same.  The 
brush-holders  B  B'  are  connected  by  means  of  the  links  e  e 
and  the  cross-piece  F  to  the  iron  core  i,  which  slides  freely  in  the 
tube  T  of  the  solenoid.  The  iron  core  i  has  a  screw,  s,  by  means 
of  which  it  can  be  raised  and  adjusted  in  its  position  relatively 
to  the  solenoid,  so  that  the  pull  exerted  upon  it  by  the  solenoid 
is  practically  uniform  through  the  whole  length  of  motion  which 
is  required  to  effect  the  regulation.  In  order  to  effect  the 
adjustment  with  greater  precision,  the  core  i  is  provided  with  a 
small  iron  screw,  s'.  The  core  being  first  brought  very  nearly 
in  the  required  position  relatively  to  the  solenoid  by  means  of 
the  screw  s,  the  small  screw  s'  is  then  adjusted  until  the  magnetic 
attraction  upon  the  core  is  the  same  when  the  core  is  in  any  posi. 
tion.  A  convenient  stop,  £,  serves  to  limit  the  upward  move- 
ment of  the  iron  core. 


44(5 


INVENTIONS  OF  NIKOLA  TESLA. 


To  check  somewhat  the  movement  of  the  core  i,  a  dash-pot,  K, 
is  used.  The  piston  L  of  the  dash-pot  is  provided  with  a  vah^e, 
v,  which  opens  by  a  downward  pressure  and  allows  an  easy- 
downward  movement  of  the  iron  core  i,  but  closes  and  checks 
the  movement  of  the  core  when  it  is  pulled  up  by  the  action 
of  the  solenoid. 

To  balance  the  opposing  forces,  the  weight  of  the  moving 
parts,  and  the  pull  exerted  by  the  solenoid  upon  the  iron  core, 
the  weights  w  w  may  be  used.  The  adjustment  is  such  that 
when  the  solenoid  is  traversed  by  the  normal  current  it  is  just 
strong  enough  to  balance  the  downward  pull  of  the  parts. 

The  electrical  circuit-connections  are  substantially  the  same  as 


FIG.  266 


indicated  in  the  previous  diagrams,  the  solenoid  being  in  series 
with  the  circuit  when  the  translating  devices  are  in  series,  and  in 
shunt  when  the  devices  are  in  multiple  arc.  The  operation  of 
the  device  is  as  follows :  When  upon  a  decrease  of  the  resis- 
tance of  the  circuit  or  for  some  other  reason,  the  current  is 
increased,  the  solenoid  s  gains  in  strength  and  pulls  up  the  iron 
core  i,  thus  shifting  the  main  brushes  in  the  direction  of  rotation 
and  the  auxiliary  brushes  in  the  opposite  way.  This  diminishes 
the  strength  of  the  current  until  the  opposing  forces  are  balanced 
and  the  solenoid  is  traversed  by  the  normal  current ;  but  if  from 
any  cause  the  current  in  the  circuit  is  diminished,  then  the  weight 
of  the  moving  parts  overcomes  the  pull  of  the  solenoid,  the  iron 


A  UXILTAR  Y  BR  U8II  RKO  ULA  TION.  447 

core  i  descends,  thus  shifting  the  brashes  the  opposite  way  and 
increasing  the  current  to  the  normal  strength.  The  dash-pot 
connected  to  the  iron  core  i  may  he  of  ordinary  construction  ; 
but  it  is  better,  especially  in  machines  for  arc  lights,  to  provide 
the  piston  of  the  dash-pot  with  a  valve,  as  indicated  in  the  dia- 
grams. This  valve  permits  a  comparatively  easy  downward  move- 
ment of  the  iron  core,  but  checks  its  movement  when  it  is  drawn 
up  by  the  solenoid.  Such  an  arrangement  has  the  advantage 
that  a  great  number  of  lights  may  be  put  on  without  diminishing 
the  light- power  of  the  lamps  in  the  circuit,  as  the  brushes  assume 
at  once  the  proper  position.  When  lights  are  cut  out,  the  dash- 
pot  acts  to  retard  the  movement ;  but  if  the  current  is  considerably 
increased  the  solenoid  gets  abnormally  strong  and  the  brushes 
are  shifted  instantly.  The  regulator  being  properly  adjusted, 
lights  or  other  devices  may  be  put  on  or  out  with  scarcely  any 
perceptible  difference.  It  is  obvious  that  instead  of  the  dash-pot 
any  other  retarding  device  may  be  used. 


CHAPTER  XXXIX. 

IMPROVEMENT  IN  THE  CONSTRUCTION  OF  DYNAMOS  AND  MOTORS. 

THIS  invention  of  Mr.  Tesla  is  an  improvement  in  the  con- 
struction of  dynamo  or  magneto  electric  machines  or  motors, 
consisting  in  a  novel  form  of  frame  and  field  magnet  which  ren- 
ders the  machine  more  solid  and  compact  as  a  structure,  which 
requires  fewer  parts,  and  which  involves  less  trouble  and  expense 
in  its  manufacture.  It  is  applicable  to  generators  and  motors 
generally,  not  only  to  those  which  have  independent  circuits 
adapted  for  use  in  the  Tesla  alternating  current  system,  but  to 
other  continuous  or  alternating  current  machines  of  the  ordinary 
type  generally  used. 

Fig.  268  shows  the  machine  in  side  elevation.  Fig.  269  is  a 
vertical  sectional  view  of  the  field  magnets  and  frame  and  an  end 
view  of  the  armature ;  and  Fig.  270  is  a  plan  view  of  one  of 
the  parts  of  the  frame  and  the  armature,  a  portion  of  the  latter 
being  cut  away. 

The  field  magnets  and  frame  are  cast  in  two  parts.  These 
parts  are  identical  in  size  and  shape,  and  each  consists  of  the  solid 
plates  or  ends  A  B,  from  which  project  inwardly  the  cores  c  D  and 
the  side  bars  or  bridge  pieces,  E  F.  The  precise  shape  of  these 
parts  is  largely  a  matter  of  choice — that  is  to  say,  each  casting, 
as  shown,  forms  an  approximately  rectangular  frame ;  but  it  might 
obviously  be  more  or  less  oval,  round,  or  square,  without  de- 
parture from  the  invention.  It  is  also  desirable  to  reduce  the 
width  of  the  side  bars,  E  F,  at  the  center  and  to  so  proportion  the 
parts  that  when  the  frame  is  put  together  the  spaces  between  the 
pole  pieces  will  be  practically  equal  to  the  arcs  which  the  sur-. 
faces  of  the  poles  occupy. 

The  bearings  G  for  the  armature  shaft  are  cast  in  the  side  bars 
E  F.  The  field  coils  are  either  wound  on  the  pole  pieces  or  on  a 
form  and  then  slipped  on  over  the  ends  of  the  pole  pieces. 
The  lower  part  or  casting  is  secured  to  the  base  after  being 
finished  off.  The  armature  K  on  its  shaft  is  then  mounted  in 


IMPROVEMENTS  IN  DYNAMOS  AND  MOTORS. 


449 


the  bearings  of  the  lower  casting  and  the  other  part  of  the  frame 
placed  in  position,  dowel  pins  L  or  any  other  means  being  used  to 
secure  the  two  parts  in  proper  position. 


FIG.  268. 


FIG.  270. 


In  order  to  secure  an  easier  fit,  the  side  bars  E  F,  and  end  pieces, 
A  B,  are  so  cast  that  slots  M  are  formed  when  the  two  parts  are 
put  together. 


450  INVENTIONS  OF  NIKOLA  TESLA. 

This  machine  possesses  several  advantages.  For  example,  if  we 
magnetize  the  cores  alternately,  as  indicated  by  the  characters  y 
s,  it  will  be  seen  that  the  magnetic  circuit  between  the  poles  of 
each  part  of  a  casting  is  completed  through  the  solid  iron  side 
bars.  The  bearings  for  the  shaft  are  located  at  the  neutral  points 
of  the  field,  so  that  the  armature  core  is  not  affected  by  the  mag- 
netic condition  of  the  field. 

The  improvement  is  not  restricted  to  the  use  of  four  pole  pieces, 
as  it  is  evident  that  each  pole  piece  could  be  divided  or  more  than 
four  formed  by  the  shape  of  the  casting. 


CHAPTER  XI, 

TKSLA   DIRECT  CURRENT  ARC  LIGHTING  SYSTKM. 

AT  one  time,  soon  after  his  arrival  in  America,  Mr.  Tesla  was 
greatly  interested  in  the  subject  of  arc  lighting,  which  then  occu- 
pied public  attention  and  readily  enlisted  the  support  of  capital. 
He  therefore  worked  out  a  system  which  was  confided  to  a  com- 
pany formed  for  its  exploitation,  and  then  proceeded  to  devote 
his  energies  to  the  perfection  of  the  details  of  his  more  celebrated 
"  rotary  field"  motor  system.  The  Tesla  arc  lighting  apparatus 
appeared  at  a  time  when  a  great  many  other  lamps  and  machines 
were  in  the  market,  but  it  commanded  notice  by  its  ingenuity. 
Its  chief  purpose  was  to  lessen  the  manufacturing  cost  and  sim- 
plify the  processes  of  operation. 

We  will  take  up  the  dynamo  first.  Fig.  271  is  a  longitudinal 
section,  and  Fig.  272  a  cross  section  of  the  machine.  Fig.  273  is 
a  top  view,  and  Fig.  274  a  side  view  of  the  magnetic  frame.  Fig. 
275  is  an  end  view  of  the  commutator  bars,  and  Fig.  276  is  a 
section  of  the  shaft  and  commutator  bars.  Fig.  277  is  a  diagram 
illustrating  the  coils  of  the  armature  and  the  connections  to  the 
commutator  plates. 

The  cores  c  c  c  c  of  the  field-magnets  are  tapering  in  both 
directions,  as  shown,  for  the  purposes  of  concentrating  the  mag- 
netism upon  the  middle  of  the  pole-pieces. 

The  connecting-frame  F  F  of  the  field-magnets  is  in  the  form 
indicated  in  the  side  view,  Fig.  274,  the  lower  part  being  pro- 
vided with  the  spreading  curved  cast  legs  e  e,  so  that  the  machine 
will  rest  firmly  upon  two  base-bars,  r  r. 

To  the  lower  pole,  s,  of  the  field-magnet  M  is  fastened,  by 
means  of  babbitt  or  other  fusible  diamagnetic  material,  the  base 
B,  which  is  provided  with  bearings  b  for  the  armature-shaft  H. 
The  base  B  has  a  projection,  p,  which  supports  the  brush-holders 
and  the  regulating  devices,  which  are  of  a  special  character  de- 
vised by  Mr.  Tesla. 

The  armature  is  constructed  with  the  view  to  reduce  to  a  min- 


453 


INVENTIONS  OF  NIKOLA   TEHLA. 


imum  the  loss  of  power  due  to  Foucault  currents  and  to  the 
change  of  polarity,  and  also  to  shorten  as  much  as  possible  the 
length  of  the  inactive  wire  wound  upon  the  armature  core. 

It  is  well  known  that  when  the  armature  is  revolved  between 
the  poles  of  the  field-magnets,  currents  are  generated  in  the  iron 
body  of  the  armature  which  develop  heat,  and  consequently  cause 


FIG.  271. 

a  waste  of  power.  Owing  to  the  mutual  action  of  the  lines  of 
force,  the  magnetic  properties  of  iron,  and  the  speed  of  the  dif- 
ferent portions  of  the  armature  core,  these  currents  are  generated 
principally  on  and  near  the  surface  of  the  armature  core,  dimin- 
ishing in  strength  gradually  toward  the  centre  of  the  core. 
Their  quantity  is  under  some  conditions  proportional  to  the 
length  of  the  iron  body  in  the  direction  in  which  these  currents 
are  generated.  By  subdividing  the  iron  core  electrically  in  this 
direction,  the  generation  of  these  currents  can  be  reduced  to  a 
great  extent.  For  instance,  if  the  length  of  the  armature-core  is 
twelve  inches,  and  by  a  suitable  construction  it  is  subdivided 
electrically,  so  that  there  are  in  the  generating  direction  six  inches 
of  iron  and  six  inches  of  intervening  air-spaces  or  insulating  ma- 
terial, the  waste  currents  will  be  reduced  to  fifty  per  cent. 

As  shown  in  the  diagrams,  the  armature  is  constructed  of  thin 
iron  discs  n  D  D,  of  various  diameters,  fastened  upon  the  arma- 
ture-shaft in  a  suitable  manner  and  arranged  according  to  their 
sizes,  so  that  a  series  of  iron  bodies,  i  i  i,  is  formed,  each  of  which 
diminishes  in  thickness  from  the  centre  toward  the  periphery. 
At  both  ends  of  the  armature  the  inwardly  curved  discs  d  d,  of 
cast  iron,  are  fastened  to  the  armature  shaft. 

The  armature  core  being  constructed  as  shown,  it  will  be  easily 
seen  that  on  those  portions  of  the  armature  that  are  the  most 
remote  from  the  axis,  and  where  the  currents  are  principally  de- 
veloped, the  length  of  iron  in  the  generating  direction  is  only  a 


DIRECT  CURRKNT  ARC  LIGHTING  SYSTEM. 


453 


small  fraction  of  the  total  length  of  the  armature  core,  and  be- 
sides this  the  iron  body  is  subdivided  in  the  generating  direction, 
and  therefore  the  Foueault  currents  are  greatly  reduced.  Another 
cause  of  heating  is  the  shifting  of  the  poles  of  the  armature  core. 
In  consequence  of  the  subdivision  of  the  iron  in  the  armature 
and  the  increased  surface  for  radiation,  the  risk  of  heating  is 
lessened. 

The  iron  discs  D  D  D  are  insulated  or  coated  with  some  insulat- 
ing-paint, a  very  careful  insulation  being  unnecessary,  as  an 
electrical  contact  between  several  discs  can  only  occur  at  places 
where  the  generated  currents  are  comparatively  weak.  An 
armature  core  constructed  in  the  manner  described  may  be  re- 
volved between  the  poles  of  the  field  magnets  without  showing 
the  slightest  increase  of  temperature. 

The  end  discs,  d  d,  which  are  of  sufficient  thickness  and,  for 
the  sake  of  cheapness,  of  cast-iron,  are  curved  inwardly,  as  in- 
dicated in  the  drawings.  The  extent  of  the  curve  is  dependent 
on  the  amount  of  wire  to  be  wound  upon  the  armatures.  In  this 
machine  the  wire  is  wound  upon  the  armature  in  two  super- 
imposed parts,  and  the  curve  of  the  end  discs,  dd,  is  so  calculated 
that  the  first  part — that  is,  practically  half  of  the  wire — just  fills 


Fro.  273. 

up  the  hollow  space  to  the  line  xx;  or,  if  the  wire  is  wound  in 
any  other  manner,  the  curve  is  such  that  when  the  whole  of  the 
wire  is  wound,  the  outside  mass  of  wires,  M>,  and  the  inside  mass 
of  wires,  w',  are  equal  at  each  side  of  the  plane  x  x.  In  this  case 
the  passive  or  electrically-inactive  wires  are  of  the  smallest 
length  practicable.  The  arrangement  has  further  the  advantage 


454 


INVENTIONS  OF  NIKOLA  TESLA. 


that  the  total   lengths  of  the  crossing  wires  at  the  two  sides  of 
the  plane  x  x  are  practically  equal. 

To  equalize  further  the  armature  coils  at  both  sides  of  the 
plates  that  are  in  contact  with  the  brushes,  the  winding  and  con- 
necting up  is  effected  in  the  following  manner :  The  whole  wire 
is  wound  upon  the  armature-core  in  two  superimposed  parts, 


which  are  thoroughly  insulated  from  each  other.  Each  of  these 
two  parts  is  composed  of  three  separated  groups  of  coils.  The 
first  group  of  coils  of  the  first  part  of  wire  being  wound  and 
connected  to  the  commutator-bars  in  the  usual  manner,  this  group 
is  insulated  and  the  second  group  wound  ;  but  the  coils  < -f  this 
second  group,  instead  of  being  connected  to  the  next  following 
commutator  bars,  are  connected  to  the  directly  opposite  bars  of 
the  commutator.  The  second  group  is  then  insulated  and  the 
third  group  wound,  the  coils  of  this  group  being  connected  to 
those  bars  to  which  they  would  be  connected  in  the  usual  way. 
The  wires  are  then  thoroughly  insulated  and  the  second  part  of 
wire  is  wound  and  connected  in  the  same  manner. 

Suppose,  for  instance,  that  there  are  twenty-four  coils — that  is, 
twelve  in  each  part — and  consequently  twenty-four  commutator 
plates.  There  will  be  in  each  part  three  groups,  each  containing 
four  coils,  and  the  coils  will  be  connected  as  follows: 

Groups.  Commutator  J><ir*\ 

(  First 1—5 

First  part  of  wire      I  Second 17 — 21 

(  Third 9—13 

(  First 13—17 

Second  part  of  wire  •<  Second 5 —  9 

(  Third 21—  1 

In  constructing  the  armature  core  and  winding  and  connecting 
the  coils  in  the  manner  indicated,  the  passive  or  electrically  in- 


DIRECT  CURRENT  ARC  LIGHTING  SYSTEM. 


455 


active  wire  is  reduced  to  a  minimum,  and  the  coils  at  each 
side  of  the  plates  that  are  in  contact  with  the  brushes  are  prac- 
tically equal.  In  this  way  the  electrical  efficiency  of  the  ma- 
chine is  increased. 

The  commutator  plates  t  are  shown  as  outside  the  bearing  b  of 


FIG.  275. 


FIG.  276. 


the  armature  shaft.  The  shaft  H  is  tubular  and  split  at  the  end 
portion,  and  the  wires  are  carried  through  the  same  in  the  usual 
manner  and  connected  to  the  respective  commutator  plates.  The 
commutator  plates  are  upon  a  cylinder,  w,  and  insulated,  and  this 
cylinder  is  properly  placed  and  then  secured  by  expanding*  the 
split  end  of  the  shaft  by  a  tapering  screw  plug,  v. 


FIG.  277. 

The  arc  lamps  invented  by  Mr.  Tesla  for  use  on  the  circuits 
from  the  above  described  dynamo  are  those  in  which  the  separa- 
tion and  feed  of  the  carbon  electrodes  or  their  equivalents  is  ac- 
complished by  means  of  electro-magnets  or  solenoids  in  connection 
with  suitable  clutch  mechanism,  and  were  designed  for  the  purpose 


4o6  INVENTIONS  OF  NIKOLA  TESLA. 

of  remedying  certain  faults  common  to  arc  lamps. 

He  proposed  to  prevent  the  frequent  vibrations  of  the  movable 
carbon  "point"  and  flickering  of  the  light  arising  therefrom;  to 
prevent  the  falling  into  contact  of  the  carbons ;  to  dispense  with 
the  dash  pot,  clock  work,  or  gearing  and  similar  devices;  to  ren- 
der the  lamp  extremely  sensitive,  and  to  feed  the  carbon  almost 
imperceptibly,  and  thereby  obtain  a  very  steady  and  uniform 
light. 

In  that  class  of  lamps  where  the  regulation  of  the  arc  is  effected 
by  forces  acting  in  opposition  on  a  free,  movable  rod  or  lever  di- 
rectly connected  with  the  electrode,  all  or  some  of  the  forces 
being  dependent  on  the  strength  of  the  current,  any  change  in 
the  electrical  condition  of  the  circuit  causes  a  vibration  and  a  cor- 
responding flicker  in  the  light.  This  difficulty  is  most  apparent 
when  there  are  only  a  few  lamps  in  circuit.  To  lessen  this  diffi- 
culty lamps  have  been  constructed  in  which  the  lever  or  armature, 
after  the  establishing  of  the  arc,  is  kept  in  a  fixed  position  and 
cannot  vibrate  during  the  feed  operation,  the  feed  mechanism 
acting  independently ;  but  in  these  lamps,  when  a  clamp  is  em- 
ployed, it  frequently  occurs  that  the  carbons  come  into  contact 
and  the  light  is  momentarily  extinguished,  and  frequently  parts 
of  the  circuit  are  injured.  In  both  these  classes  of  lamps  it  has 
been  customary  to  use  dash  pot,  clock  work,  or  equivalent  retard- 
ing devices ;  but  these  are  often  unreliable  and  objectionable,  and 
increase  the  cost  of  construction. 

Mr.  Tesla  combines  two  electro-magnets — one  of  low  resis- 
tance in  the  main  or  lamp  circuit,  and  the  other  of  comparatively 
high  resistance  in  a  shunt  around  the  arc — a  movable  armature 
lever,  and  a  special  feed  mechanism,  the  parts  being  arranged  so 
that  in  the  normal  working  position  of  the  armature  lever  the 
same  is  kept  almost  rigidly  in  one  position,  and  is  not  affected 
even  by  considerable  changes  in  the  electric  circuit ;  but  if  the 
carbons  fall  into  contact  the  armature  will  be  actuated  by  the 
magnets  so  as  to  move  the  lever  and  start  the  arc,  and  hold  the 
carbons  until  the  arc  lengthens  and  the  armature  lever  returns  to 
the  normal  position.  After  this  the  carbon  rod  holder  is  released 
by  the  action  of  the  feed  mechanism,  so  as  to  feed  the  carbon  and 
restore  the  arc  to  its  normal  length. 

Fig.  278  is  an  elevation  of  the  mechanism  made  use  of  in 
this  arc  lamp.  Fig.  279  is  a  plan  view.  Fig.  280  is  an  ele- 
vation of  the  balancing  lever  and  spring;  Fig.  281  is  a  de- 


DIRECT  CURRENT  ARC  LIGHTING  8YSTKM. 


4f>7 


taclied  plan  view  of  the   pole  pieces  and  armatures  upon   the 
friction  clamp,  and  Fig.  282  is  a  section  of  the  clamping  tube. 

M  is  a  helix  of  coarse  wire  in  a  circuit  from  the  lower  carbon 
holder  to  the  negative  binding  screw — .  N  is  a  helix  of  fine  wire 
in  a  shunt  between  the  positive  binding  screw  -\-  and  the 
negative  binding  screw  — .  The  upper  carbon  holder  s  is  a  paral- 
lel rod  sliding  through  the  plates  s'  s2  of  the  frame  of  the  lamp, 
and  hence  the  electric  current  passes  from  the  positive  binding 


FIG.  279, 


FIG.  281. 


FIG.  280. 


post  _j_  through  the  plate  s2,  carbon  holder  s,  and  upper  carbon 
to  the  lower  carbon,  and  thence  by  the  holder  and  a  metallic 
connection  to  the  helix  M. 

The  carbon  holders  are  of  the  usual  character,  and  to  insure 
electric  connections  the  springs  I  are  made  use  of  to  grasp  the 
upper  carbon  holding  rod  s,  but  to  allow  the  rod  to  slide  freely 
through  the  same.  These  springs  /  may  be  adjusted  in  their 
pressure  by  the  screw  m,  and  the  spring  /  may  be  sustained  upon 


4.->s  INVENTIONS  OF  NIKOLA  TE8LA. 

any  suitable  support.  They  are  shown  as  connected  with  the 
upper  end  of  the  core  of  the  magnet  N. 

Around  the  carbon-holding  rod  s,  between  the  plates  s'  s2? 
there  is  a  tube,  R,  which  forms  a  clamp.  This  tube  is  counter- 
bored,  as  seen  in  the  section  Fig.  282,  so  that  it  bears  upon  the 
rod  s  at  its  upper  end  and  near  the  middle,  and  at  the  lower  end  of 
this  tubular  clamp  K  there  are  armature  segments  r  of  soft  iron. 
A  frame  or  arm,  n,  extending,  preferably,  from  the  core  N2,  sup- 
ports the  lever  A  by  a  fulcrum-pin,  o.  This  lever  A  has  a  hole, 
through  which  the  upper  end  of  the  tubular  clamp  E  passes 
freely,  and  from  the  lever  A  is  a  link,  q,  to  the  lever  2,  which 
lever  is  pivoted  at  y  to  a  ring  upon  one  of  the  columns  s8.  This 
lever  t  has  an  opening  or  bow  surrounding  the  tubular  clamp 
K,  and  there  are  pins  or  pivotal  connections  w  between  the  lever 
t  and  this  clamp  R,  and  a  spring,  r,  serves  to  support  or  suspend 
the  weight  of  the  parts  and  balance  them,  or  nearly  so.  This 
spring  is  adjustable. 

At  one  end  of  the  lever  A  is  a  soft-iron  armature  block,  «,  over 
the  core  M'  of  the  helix  M,  and  there  is  a  limiting  screw,  c,  pass- 
ing through  this  armature  block  «,  and  at  the  other  end  of  the 
lever  A  is  a  soft  iron  armature  block,  5,  with  the  end  tapering  or 
wedge  shaped,  and  the  same  comes  close  to  and  in  line  with  the 
lateral  projection  e  on  the  core  N2.  The  lower  ends  of  the  cores 
M'  N2  are  made  with  laterally  projecting  pole-pieces  M3  N3,  respect- 
ively, and  these  pole-pieces  are  concave  at  their  outer  ends,  and 
are  at  opposite  sides  of  the  armature  segments  ;•  at  the  lower  end 
of  the  tubular  clamp  R. 

The  operation  of  these  devices  is  as  follows  :  In  the  condition 
of  inaction,  the  upper  carbon  rests  upon  the  lower  one,  and  when 
the  electric  current  is  turned  on  it  passes  freely,  by  the  frame 
and  spring  /,  through  the  rods  and  carbons  to  the  coarse  wire  and 
helix  M,  and  to  the  negative  binding  post  v  and  the  core  M'  thereby 
is  energized.  The  pole  piece  M3  attracts  the  armature  r,  and  by 
the  lateral  pressure  causes  the  clamp  R  to  grasp  the  rod  s',  and 
the  lever  A  is  simultaneously  moved  from  the  position  shown  by 
dotted  lines,  Fig.  278,  to  the  normal  position  shown  in  full  lines, 
and  in  so  doing  the  link  q  and  lever  t  are  raised,  lifting  the  clamp 
R  and  s,  separating  the  carbons  and  forming  the  arc.  The  mag- 
netism of  the  pole  piece  e  tends  to  hold  the  lever  A  level,  or 
nearly  so,  the  core  N2  being  energized  by  the  current  in  the  shunt 
which  contains  the  helix  N.  In  this  position  the  lever  A  is  not 


DIRECT  CUKRKNT  ABC  LIGHTING  SYSTEM.  4.V.) 

moved  by  any  ordinary  variation  in  the  current,  because  the  arm- 
ature b  is  strongly  attracted  by  the  magnetism  of  <?,  and  these 
parts  are  close  to  each  other,  and  the  magnetism  of  e  acts  at  right 
angles  to  the  magnetism  of  the  core  M'.  If,  now,  the  arc  becomes 
too  long,  the  current  through  the  helix  M  is  lessened,  and  the  mag- 
netism of  the  core  N3  is  increased  by  the  greater  current  passing 
through  the  shunt,  and  this  core  N3,  attracting  the  segmental  arm- 
ature /•,  lessens  the  hold  of  the  clamp  R  upon  the  rod  s,  allowing 
the  latter  to  slide  and  lessen  the  length  of  the  arc,  which  instantly 
restores  the  magnetic  equilibrium  and  causes  the  clamp  R  to  hold 
the  rod  s.  If  it  happens  that  the  carbons  fall  into  contact,  then 
the  magnetism  of  N2  is  lessened  so  much  that  the  attraction  of 
the  magnet  M  will  be  sufficient  to  move  the  armature  a  and  lever 
A  so  that  the  armature  b  passes  above  the  normal  position,  so  as 
to  separate  the  carbons  instantly;  but  when  the  carbons  burn 
away,  a  greater  amount  of  current  will  pass  through  the  shunt 
until  the  attraction  of  the  core  N2  will  overcome  the  attraction  of 
the  core  M'  and  bring  the  armature  lever  A  again  into  the  normal 
horizontal  position,  and  this  occurs  before  the  feed  can  take  place. 
The  segmental  armature  pieces  '/•  are  shown  as  nearly  semicircular. 
They  are  square  or  of  any  other  desired  shape,  the  ends  of  the 
pole  pieces  M3,  N3  being  made  to  correspond  in  shape. 

In  a  modification  of  this  lamp,  Mr.  Tesla  provided  means  for 
automatically  withdrawing  a  lamp  from  the  circuit,  or  cutting 
it  out  when,  from  a  failure  of  the  feed,  the  arc  reached  an 
abnormal  length ;  and  also  means  for  automatically  reinserting 
such  lamp  in  the  circuit  when  the  rod  drops  and  the  carbons 
come  into  contact. 

Fig.  283  is  an  elevation  of  the  lamp  with  the  case  in  section. 
Fig.  284  is  a  sectional  plan  at  the  line  x  .r.  Fig.  285  is  an  ele- 
vation, partly  in  section,  of  the  lamp  at  right  angles  to  Fig.  283. 
Fig.  286  is  a  sectional  plan  at  the  line  y  y  of  Fig.  283.  Fig.  287 
is  a  section  of  the  clamp  in  about  full  size.  Fig.  288  is  a  de- 
tached section  illustrating  the  connection  of  the  spring  to  the 
lever  that  carries  the  pivots  of  the  clamp,  and  Fig.  289  is  a 
diagram  showing  the  circuit-connections  of  the  lamp. 

In  Fig.  283,  M  represents  the  main  and  N  the  shunt  magnet,  both 
securely  fastened  to  the  base  A,  which  with  its  side  columns,  s  s, 
are  cast  in  one  piece  of  brass  or  other  diamagnetic  material.  To 
the  magnets  are  soldered  or  otherwise  fastened  the  brass  washers 
or  discs  a  a  a  a.  Similar  washers,  b  &,  of  fibre  or  other  insii- 


460 


IKVKNTIOXS  OF  NIKOLA   TKSLA. 


lating  material,  serve  to  insulate  the  wires  from  the  brass  washers. 
The  magnets  M  and  N  are  made  very  flat,  so  that  their  width 
exceeds  three  times  their  thickness,  or  even  more.  In  this  way 
a  comparatively  small  number  of  convolutions  is  sufficient  to  pro- 
duce the  required  magnetism,  while  a  greater  surface  is  offered 
for  cooling  off  the  wires. 


FIG.  284. 


FIG.  287.        FIG.  288. 


The  upper  pole  pieces,  ///  »,  of  the  magnets  are  curved,  as  in- 
dicated in  the  drawings,  Fig.  283.  The  lower  pole  pieces///  /i', 
are  brought  near  together,  tapering  toward  the  armature  g,  as 
shown  in  Figs.  284  and  286.  The  object  of  this  taper  is  to  con- 
centrate the  greatest  amount  of  the  developed  magnetism  upon 
the  armature,  and  also  to  allow  the  pull  to  be  exerted  always  upon 
the  middle  of  the  armature  y.  This  armature  yisa  piece  of  iron 


DIRECT  CURRKNT  ARC  LIGHTING  SYSTEM.  461 

in  the  shape  of  a  hollow  cylinder,  having  on  each  side  a  segment 
cut  away,  the  width  of  which  is  equal  to  the  width  of  the  pole 
pieces  m'  n'. 

The  armature  is  soldered  or  otherwise  fastened  to  the  clamp  /-, 
which  is  formed  of  a  brass  tube,  provided  with  gripping-jaws  e 
Fig.  287.  These  jaws  are  arcs  of  a  circle  of  the  diameter  of  the 
rod  R,  and  are  made  of  hardened  German  silver.  The  guides 
/"/',  through  which  the  carbon-holding  rod  E  slides,  are  made  of 
the  same  material.  This  has  the  advantage  of  reducing  greatly  the 
wear  and  corrosion  of  the  parts  coming  in  frictional  contact  with 
the  rod,  which  frequently  causes  trouble.  The  jaws  e  e  are 
fastened  to  the  inside  of  the  tube  r,  so  that  one  is  a  little  lower 
than  the  other.  The  object  of  this  is  to  provide  a  greater  open- 
ing for  the  passage  of  the  rod  when  the  same  is  released  by  the 
clamp.  The  clamp  r  is  supported  on  bearings  w  w,  Figs.  283, 
285  and  287,  which  are  just  in  the  middle  between  the  jaws  e  e. 
The  bearings  w  w  are  carried  by  a  lever,  t,  one  end  of  which 
rests  upon  an  adjustable  support,  §-,  of  the  side  columns,  s,  the 
other  end  being  connected  by  means  of  the  link  e'  to  the  arma- 
ture-lever L.  The  armature-lever  L  is  a  flat  piece  of  iron  in  |sj 
shape,  having  its  ends  curved  so  as  to  correspond  to  the  form  of 
the  upper  pole-pieces  of  the  magnets  M  and  N.  It  is  hung  upon 
the  pivots  v  v,  Fig.  284,  which  are  in  the  jaw  x  of  the 
top  plate  B.  This  plate  B,  with  the  jaw,  is  cast  in  one  piece 
and  screwed  to  the  side  columns,  s  s,  that  extend  up  from  the 
base  A.  To  partly  balance  the  overweight  of  the  moving  parts, 
a  spring,  «',  Figs.  284  and  288,  is  fastened  to  the  top  plate,  B, 
and  hooked  to  the  lever  t.  The  hook  o  is  toward  one  side  of  the 
lever  or  bent  a  little  sidewise,  as  seen  in  Fig.  288.  By  this  means 
a  slight  tendency  is  given  to  swing  the  armature  toward  the 
pole-piece  m'  of  the  main  magnet. 

The  binding-posts  K  K'  are  screwed  to  the  base  A.  A  manual 
switch,  for  short-circuiting  the  lamp  when  the  carbons  are  re- 
newed, is  also  fastened  to  the  base.  This  switch  is  of  ordinary 
character,  and  is  not  shown  in  the  drawings. 

The  rod  E  is  electrically  connected  to  the  lamp-frame  by  means 
of  a  flexible  conductor  or  otherwise.  The  lamp-case  receives  a 
removable  cover,  s2,  to  inclose  the  parts. 

The  electrical  connections  are  as  indicated  diagrammatically  in 
Fig.  289.  The  wire  in  the  main  magnet  consists  of  two  parts, 
a?'  and  p'.  These  two  parts  may  be  in  two  separated  coils  or  in 


4(52 


INVENTIONS  OF  NIKOLA   TKSLA. 


one  single  helix,  as  shown  in  the  drawings.  The  part  ,//  being 
normally  in  circuit,  is,  with  the  fine  wire  upon  the  shunt-magnet, 
wound  and  traversed  by  the  current  in  the  same  direction,  so  as 
to  tend  to  produce  similar  poles,  N  N  or  s  s,  on  the  corresponding 
pole-pieces  of  the  magnets  M  and  N.  The  part  p'  is  only  in  cir- 
cuit when  the  lamp  is  cut  out,  and  then  the  current  being  in  the 
opposite  direction  produces  in  the  main  magnet,  magnetism  of 
the  opposite  polarity. 

The  operation  is  as  follows  :  At  the  start  the  carbons  are  to 
be  in  contact,  and  the  current  passes  from  the  positive  binding- 
post  K  to  the  lamp-frame,  carbon-holder,  upper  and  lower  carbon, 
insulated  return-  wire  in  one  of  the  side  rods,  and  from  there 
through  the  part  x'  of  the  wire  on  the  main  magnet  to  the  nega- 


Fro.  289. 

tive  binding-post.  Upon  the  passage  of  the  current  the  main 
magnet  is  energized  and  attracts  the  clamping-armature  g,  swing- 
ing the  clamp  and  gripping  the  rod  by  means  of  the  gripping 
jaws  e  e.  At  the  same  time  the  armature  lever  L  is  pulled  down 
and  the  carbons  are  separated.  In  pulling  down  the  armature  lever 
L  the  main  magnet  is  assisted  by  the  shunt-magnet  N,  the  latter 
being  magnetized  by  magnetic  induction  from  the  magnet  M. 

Tt  will  be  seen  that  the  armatures  L  and  g  are  practically  the 
keepers  for  the  magnets  M  and  N,  and  owing  to  this  fact  both 
magnets  with  either  one  of  the  armatures  L  and  g  may  be  con- 
sidered as  one  horseshoe  magnet,  which  we  might  term  a  "  com- 
pound magnet."  The  whole  of  the  soft-iron  parts  M,  m't  g,  n'y 
N  and  i,  form  a  compound  magnet. 


DIRECT  CURRENT  ARC  LIGHTING  SYXTKM.  4fc] 

The  carbons  being  separated,  the  fine  wire  receives  a  portion 
of  the  current.  Now,  the  magnetic  induction  from  the  magnet 
M  is  such  as  to  produce  opposite  poles  on  the  corresponding  ends 
of  the  magnet  N  ;  but  the  current  traversing  the  helices  tends  to 
produce  similar  poles  on  the  corresponding  ends  of  both  magnets, 
and  therefore  as  soon  as  the  fine  wire  is  traversed  by  sufficient 
current  the  magnetism  of  the  whole  compound  magnet  is  dimin- 
ished. 

With  regard  to  the  armature  g  and  the  operation  of  the  lamp, 
the  pole  77i '  may  be  considered  as  the  "  clamping  "  and  the  pole  /// 
as  the  "  releasing  "  pole. 

As  the  carbons  burn  away,  the  fine  wire  receives  more  current 
and  the  magnetism  diminishes  in  proportion.  This  causes  the 
armature  lever  L  to  swing  and  the  armature  g  to  descend  grad- 
ually under  the  weight  of  the  moving  parts  until  the  end/>,  Fig. 
283,  strikes  a  stop  on  the  top  plate,  B.  The  adjustment  is  such 
that  when  this  takes  place  the  rod  K  is  yet  gripped  securely  by 
the  jaws  ee.  The  further  downward  movement  of  the  armature 
lever  being  prevented,  the  arc  becomes  longer  as  the  carbons  are 
consumed,  and  the  compound  magnet  is  weakened  more  and 
more  until  the  clamping  armature  g  releases  the  hold  of  the 
gripping-jaws  e  e  upon  the  rod  R,  and  the  rod  is  allowed  to  drop 
a  little,  thus  shortening  the  arc.  The  fine  wire  now  receiving 
less  current,  the  magnetism  increases,  and  the  rod  is  clamped 
again  and  slightly  raised,  if  necessary.  This  clamping  and  re- 
leasing of  the  rod  continues  until  the  carbons  are  consumed.  In 
practice  the  feed  is  so  sensitive  that  for  the  greatest  part  of  the 
time  the  movement  of  the  rod  cannot  be  detected  without  some 
actual  measurement.  During  the  normal  operation  of  the  lamp 
the  armature  lever  L  remains  practically  stationary,  in  the  posi- 
tion showTn  in  Fig.  283. 

Should  it  happen  that,  owing  to  an  imperfection  in  it,  the  rod 
and  the  carbons  drop  too  far,  so  as  to  make  the  arc  too  short,  or 
even  bring  the  carbons  in  contact,  a  very  small  amount  of  cur- 
rent passes  through  the  fine  wire,  and  the  compound  magnet 
becomes  sufficiently  strong  to  act  as  at  the  start  in  pulling  the 
armature  lever  L  down  and  separating  the  c'arbons  to  a  greater 
distance. 

It  occurs  often  in  practical  work  that  the  rod  sticks  in  the 
guides.  In  this  case  the  arc  reaches  a  great  length,  until  it  finally 
breaks.  Then  the  light  goes  out,  and  frequently  the  fine  wire  is 


464  INVENTIONS  OF  NIKOLA  TESLA, 

injured.  To  prevent  such  an  accident  Mr.  Tesla  provides  this 
lamp  with  an  automatic  cut-out  which  operates  as  follows  :  When, 
upon  a  failure  of  the  feed,  the  arc  reaches  a  certain  predeter- 
mined length,  such  an  amount  of  current  is  diverted  through 
the  fine  wire  that  the  polarity  of  the  compound  magnet  is  re- 
versed. The  clamping  armature  g  is  now  moved  against  the 
shunt  magnet  N  until  it  strikes  the  releasing  pole  n'.  As  soon 
as  the  contact  is  established,  the  current  passes  from  the  positive 
binding  post  over  the  clamp  />,  armature  g,  insulated  shunt  mag- 
net, and  the  helix  p'  upon  the  main  magnet  M  to  the  negative 
binding  post.  In  this  case  the  current  passes  in  the  opposite  di- 
rection and  changes  the  polarity  of  the  magnet  M,  at  the  same 
time  maintaining  by  magnetic  induction  in  the  core  of  the  shunt 
magnet  the  required  magnetism  without  reversal  of  polarity,  and 
the  armature  g  remains  against  the  shunt  magnet  pole  n'.  The 
lamp  is  thus  cut  out  as  long  as  the  carbons  are  separated.  The 
cut  out  may  be  used  in  this  form  without  any  further  improve- 
ment ;  but  Mr.  Tesla  arranges  it  so  that  if  the  rod  drops  and  the 
carbons  come  in  contact  the  arc  is  started  again.  For  this  pur- 
pose he  proportions  the  resistance  of  part  j!/  and  the  number  of 
the  convolutions  of  the  wire  upon  the  main  magnet  so  that  when 
the  carbons  come  in  contact  a  sufficient  amount  of  current  is  di- 
verted through  the  carbons  and  the  part  x'  to  destroy  or  neutral- 
ize the  magnetism  of  the  compound  magnet,  Then  the  arma- 
ture g,  having  a  slight  tendency  to  approach  to  the  clamping  pole 
m't  comes  out  of  contact  with  the  releasing  pole  n'.  As  soon  as 
this  happens,  the  current  through  the  part  j?'  is  interrupted,  and 
the  whole  current  passes  through  the  part  x.  The  magnet  M  is 
now  strongly  magnetized,  the  armature  g  is  attracted,  and  the 
rod  clamped.  At  the  same  time  the  armature  lever  L  is  pulled 
down  out  of  its  normal  position  and  the  arc  started.  In  this  way 
the  lamp  cuts  itself  out  automatically  when  the  arc  gets  too  long, 
and  reinserts  itself  automatically  in  the  circuit  if  the  carbons  drop 
together. 


CHAPTER  XLI. 

IMPROVEMENT  IN   "UNIPOLAR"    GENERATORS. 

ANOTHER  interesting  class  of  apparatus  to  which  Mr.  Tesla  has 
directed  his  attention,  is  that  of  "  unipolar  "  generators,  in  which  a 
disc  or  a  cylindrical  conductor  is  mounted  between  magnetic 
poles  adapted  to  produce  an  approximately  uniform  field.  In 
the  disc  armature  machines  the  currents  induced  in  the  rotating 
conductor  flow  from  the  centre  to  the  periphery,  or  conversely, 
according  to  the  direction  of  rotation  or  the  lines  of  force  as  de- 
termined by  the  signs  of  the  magnetic  poles,  and  these  currents 
are  taken  off  usually  by  connections  or  brushes  applied  to  the 
disc  at  points  on  its  periphery  and  near  its  centre.  In  the  case 
of  the  cylindrical  armature  machine,  the  currents  developed  in 
the  cylinder  are  taken  off  .by  brushes  applied  to  the  sides  of  the 
cylinder  at  its  ends. 

In  order  to  develop  economically  an  electromotive  force  avail- 
able for  practicable  purposes,  it  is  necessary  either  to  rotate  the 
conductor  at  a  very  high  rate  of  speed  or  to  use  a  disc  of  large 
diameter  or  a  cylinder  of  great  length ;  but  in  either  case  it  be- 
comes difficult  to  secure  and  maintain  a  good  electrical  connection 
between  the  collecting  brushes  and  the  conductor,  owing  to  the 
high  peripheral  speed. 

It  has  been  proposed  to  couple  two  or  more  discs  together  in 
series,  with  the  object  of  obtaining  a  higher  electro-motive  force  ; 
but  with  the  connections  heretofore  used  and  using  other  condi- 
tions of  speed  and  dimension  of  disc  necessary  to  securing  good 
practicable  results,  this  difficulty  is  still  felt  to  be  a  serious 
obstacle  to  the  use  of  this  kind  of  generator.  These  objections 
Mr.  Tesla  has  sought  to  avoid  by  constructing  a  machine  with 
two  fields,  each  having  a  rotary  conductor  mounted  between  its 
poles.  The  same  principle  is  involved  in  the  case  of  both  forms 
of  machine  above  described,  but  the  description  now  given  is 
confined  to  the  disc  type,  which  Mr.  Tesla  is  inclined  to  favor  for 
that  machine.  The  discs  are  formed  with  flanges,  after  tho 


466 


INVENTIONS  OF  NIKOLA  TESLA. 


manner  of  pulleys,  and  are  connected  together  by  flexible  con- 
ducting bands  or  belts. 

The  machine  is  built  in  such  manner  that  the  direction  of 
magnetism  or  order  of  the  poles  in  one  tield  of  force  is  opposite 
to  that  in  the  other,  so  that  rotation  of  the  discs  in  the  same  di- 
rection develops  a  current  in  one  from  centre  to  circumference 
and  in  the  other  from  circumference  to  centre.  Contacts  applied 
therefore  to  the  shafts  upon  which  the  discs  are  mounted  form 
the  terminals  of  a  circuit  the  electro-motive  force  in  which  is  the 
sum  of  the  electro-motive  forces  of  the  two  dises. 

It  will  be  obvious  that  if  the  direction  of  magnetism  in  both 


Fro.  290. 


FIG.  291. 


fields  be  the  same,  the  same  result  as  above  will  be  obtained  by 
driving  the  discs  in  opposite  directions  and  crossing  the  connect- 
ing belts.  In  this  way  the  difficulty  of  securing  and  maintaining 
good  contact  with  the  peripheries  of  the  discs  is  avoided  and  a 
cheap  and  durable  machine  made  which  is  useful  for  many  pur- 
poses— such  as  for  an  exciter  for  alternating  current  generators, 
for  a  motor,  and  for  any  other  purpose  for  which  dynamo  ma- 
chines are  used. 

Fig.  290  is  a  side  view,  partly  in  section,  of  this  machine. 
Fig.  291  is  a  vertical  section  of  the  same  at  right  angles  to  the 
shafts. 


UNIPOLAR  GENERATORS.  467 

In  order  to  form  a  frame  with  two  fields  of  force,  a  support, 
A,  is  cast  with  two  pole  pieces  u  B'  integral  with  it.  To  this  are 
joined  by  bolts  E  a  casting  D,  with  two  similar  and  corresponding 
pole  pieces  c  c'.  The  pole  pieces  B  B'  are  wound  and  connected 
to  produce  a  field  of  force  of  given  polarity,  and  the  pole 
pieces  c  c'  are  wound  so  as  to  produce  a  field  of  opposite  po- 
larity. The  driving  shafts  F  G  pass  through  the  poles  and  are 
journaled  in  insulating  bearings  in  the  casting  A  u,  as  shown. 

H  K  are  the  discs  or  generating  conductors.  They  are  com- 
posed of  copper,  brass,  or  iron  and  are  keyed  or  secured  to  their  re- 
spective shafts.  They  are  provided  with  broad  peripheral  flanges 
j.  It  is  of  course  obvious  that  the  discs  may  be  insulated  from  their 
shafts,  if  so  desired.  A  flexible  metallic  belt  L  is  passed  over  the 
flanges  of  the  two  discs,  and,  if  desired,  maybe  used  to  drive  one 
of  the  discs.  It  is  better,  however,  to  use  this  belt  merely  as  a 
conductor,  and  for  this  purpose  sheet  steel,  copper,  or  other  suit- 
able metal  is  used.  Each  shaft  is  provided  with  a  driving  pulley 
M,  by  which  power  is  imparted  from  a  driving  shaft. 

N  N  are  the  terminals.  For  the  sake  of  clearness  they  are  shown 
as  provided  with  springs  p,  that  bear  upon  the  ends  of  the  shafts. 
This  machine,  if  self-exciting,  would  have  copper  bands  around 
its  poles  ;  or  conductors  of  any  kind — such  as  wires  shown  in 
thexlrawings — may  be  used. 


It  is  thought  appropriate  by  the  compiler  to  append  here  some 
notes  on  unipolar  dynamos,  written  by  Mr.  Tesla,  on  a  recent  oc- 
casion. 


It  is  characteristic  of  fundamental  discoveries,  of  great  achieve- 
ments of  intellect,  that  they  retain  an  undiminished  power  upon 
the  imagination  of  the  thinker.  The  memorable  experiment  of 
Faraday  with  a  disc  rotating  between  the  two  poles  of  a  magnet, 
which  has  borne  such  magnificent  fruit,  has  long  passed  into 
every-day  experience ;  yet  there  are  certain  features  about  this 
embryo  of  the  present  dynamos  and  motors  which  even  to-day 
appear  to  us  striking,  and  are  worthy  of  the  most  careful  study. 

Consider,  for  instance,  the  case  of  a  disc  of  iron  or  other  metal 

1.  Article  by  Mr.  Tesla,  contributed  to  The  Electrical  Engineer,  N.  Y., 
Sept.  2,  1891. 


468  INVENTIONS  OF  NIKOLA  TESLA. 

revolving  between  the  two  opposite  poles  of  a  magnet,  and  the 
polar  surfaces  completely  covering  both  sides  of  the  disc,  and 
assume  the  current  to  be  taken  off  or  conveyed  to  the  same  by 
contacts  uniformly  from  all  points  of  the  periphery  of  the  disc. 
Take  first  the  case  of  a  motor.  In  all  ordinary  motors  the  opera- 
tion is  dependent  upon  some  shifting  or  change  of  the  resultant 
of  the  magnetic  attraction  exerted  upon  the  armature,  this  pro- 
cess being  effected  either  by  some  mechanical  contrivance  on  the 
motor  or  by  the  action  of  currents  of  the  proper  character.  We 
may  explain  the  operation  of  such  a  motor  just  as  we  can  that  of 
a  water-wheel.  But  in  the  above  example  of  the  disc  surrounded 
completely  by  the  polar  surfaces,  there  is  no  shifting  of  the  mag- 
netic action,  no  change  whatever,  as  far  as  we  know,  and  yet 
rotation  ensues.  Here,  then,  ordinary  considerations  do  not 
apply  ;  we  cannot  even  give  a  superficial  explanation,  as  in  ordi- 
nary motors,  and  the  operation  will  be  clear  to  us  only  when  we 
shall  have  recognized  the  very  nature  of  the  forces  concerned, 
and  fathomed  the  mystery  of  the  invisible  connecting  mechan- 
ism. 

Considered  as  a  dynamo  machine,  the  disc  is  an  equally  inter- 
esting object  of  study.  In  addition  to  its  peculiarity  of  giving 
currents  of  one  direction  without  the  employment  of  commutat- 
ing  devices,  such  a  machine  differs  from  ordinary  dynamos  in 
that  there  is  no  reaction  between  armature  and  field.  The  arma- 
ture current  tends  to  set  up  a  magnetization  at  right  angles  to 
that  of  the  field  current,  but  since  the  current  is  taken  off  uni- 
formly from  all  points  of  the  periphery,  and  since,  to  be  exact, 
the  external  circuit  may  also  be  arranged  perfectly  symmetrical 
to  the  field  magnet,  no  reaction  can  occur.  This,  however,  is 
true  only  as  long  as  the  magnets  are  weakly  energized,  for  when 
the  magnets  are  more  or  less  saturated,  both  magnetizations  at 
right  angles  seemingly  interfere  with  each  other. 

For  the  above  reason  alone  it  would  appear  that  the  output  of 
such  a  machine  should,  for  the  same  weight,  be  much  greater 
than  that  of  any  other  machine  in  which  the  armature  current 
tends  to  demagnetize  the  field.  The  extraordinary  output  of  the 
Forbes  unipolar  dynamo  and  the  experience  of  the  writer  con- 
firm this  view. 

Again,  the  facility  with  which  such  a  machine  may  be  made  to 
excite  itself  is  striking,  but  this  may  be  due — besides  to  the  ab- 
sence of  armature  reaction — to  the  perfect  smoothness  of  the  cur- 
rent and  non-existence  of  self-induction. 


UNIPOLAR  GENERATORS. 


469 


If  the  poles  do  not  cover  the  disc  completely  on  both  sides, 
then,  of  course,  unless  the  disc  be  properly  subdivided,  the 
machine  will  be  very  inefficient.  Again,  in  this  case  there  are 
points  worthy  of  notice.  If  the  disc  be  rotated  and  the  field 
current  interrupted,  the  current  through  the  armature  will  con- 
tinue to  flow  and  the  field  magnets  will  lose  their  strength  com- 
paratively slowly.  The  reason  for  this  will  at  once  appear  when 
we  consider  the  direction  of  the  currents  set  up  in  the  disc. 

Referring  to  the  diagram  Fig.  292,  d  represents  the  disc  with 
the  sliding  contacts  B  B'  on  the  shaft  and  periphery.  N  and  s 
represent  the  two  poles  of  a  magnet.  If  the  pole  N  be  above,  as 
indicated  in  the  diagram,  the  disc  being  supposed  to  be  in  the 


Fio.  292. 

plane  of  the  paper,  and  rotating  in  the  direction  of  the  arrow  D, 
the  current  set  up  in  the  disc  will  flow  from  the  centre  to  the 
periphery,  as  indicated  by  the  arrow  A.  Since  the  magnetic  ac- 
tion is  more  or  less  confined  to  the  space  between  the  poles  N  s, 
the  other  portions  of  the  disc  may  be  considered  inactive.  The 
current  set  up  will  therefore  not  wholly  pass  through  the  external 
circuit  F,  but  will  close  through  the  disc  itself,  and  generally,  if 
the  disposition  be  in  any  way  similar  to  the  one  illustrated,  by  far 
the  greater  portion  of  the  current  generated  will  not  appear  ex- 
ternally, as  the  circuit  F  is  practically  short-circuited  by  the  inac- 
tive portions  of  the  disc.  The  direction  of  the  resulting  currents 
in  the  latter  may  be  assumed  to  be  as  indicated  by  the  dotted 


470  INVENTIONS  OF  NIKOLA  TESLA. 

lines  and  arrows  HI  and  n  /  and  tlie  direction  of  the  energizing 
field  current  being  indicated  by  the  arrows  a  b  c  d,  an  inspection  of 
the  figure  shows  that  one  of  the  two  branches  of  the  eddy  current-, 
that  is,  A  B'  m  B,  will  tend  to  demagnetize  the  field,  while  the 
other  branch,  that  is,  A  B'  n  B,  will  have  the  opposite  effect. 
Therefore,  the  branch  A  B'  m  B,  that  is,  the  one  which  is  approach- 
ing the  field,  will  repel  the  lines  of  the  same,  while  branch  A  B' 
n  B,  that  is,  the  one  leaving  the  field,  will  gather  the  lines  of 
force  upon  itself. 

In  consequence  of  this  there  will  be  a  constant  tendency  to 
reduce  the  current  flow  in  the  path  A  B'  m  B,  while  on  the  other 
hand  no  such  opposition  will  exist  in  path  A  B'  n  B,  and  the  effect 
of  the  latter  branch  or  path  will  be  more  or  less  preponderating 
over  that  of  the  former.  The  joint  effect  of  both  the  assumed 
branch  currents  might  be  represented  by  that  of  one  single  cur- 
rent of  the  same  direction  as  that  energizing  the  field.  In  other 
words,  the  eddy  currents  circulating  in  the  disc  will  energize  the 
field  magnet.  This  is  a  result  quite  contrary  to  what  we  might 
be  led  to  suppose  at  first,  for  we  would  naturally  expect  that  the 
resulting  effect  of  the  armature  currents  would  be  such  as  to 
oppose  the  field  current,  as  generally  occurs  when  a  primary  and 
secondary  conductor  are  placed  in  inductive  relations  to  each 
other.  But  it  must  be  remembered  that  this  results  from  the 
peculiar  disposition  in  this  case,  namely,  two  paths  being  afforded 
to  the  current,  and  the  latter  selecting  that  path  which  offers  the 
least  opposition  to  its  flow.  From  this  we  see  that  the  eddy 
currents  flowing  in  the  disc  partly  energize  the  field,  and  for  this 
reason  when  the  field  current  is  interrupted  the  currents  in  the 
disc  will  continue  to  flow,  and  the  field  magnet  will  lose  its 
strength  with  comparative  slowness  and  may  even  retain  a  cer- 
tain strength  as  long  as  the  rotation  of  the  disc  is  continued. 

The  result  will,  of  course,  largely  depend  on  the  resistance 
and  geometrical  dimensions  of  the  path  of  the  resulting  eddy 
current  and  on  the  speed  of  rotation ;  these  elements,  namely, 
determine  the  retardation  of  this  current  and  its  position  relative 
to  the  field.  For  a  certain  speed  there  would  be  a  maximum 
energizing  action ;  then  at  higher  speeds,  it  would  gradually  fall 
off  to  zero  and  finally  reverse,  that  is,  the  resultant  eddy  current 
effect  would  be  to  weaken  the  field.  The  reaction  would  be 
best  demonstrated  experimentally  by  arranging  the  fields  N  s, 
N'  s',  freely  movable  on  an  axis  concentric  with  the  shaft  of  the 


UNIPOLAR  GENERATORS.  471 

disc.  If  the  latter  were  rotated  as  before  in  the  direction  of  the 
arrow  D,  the  field  would  be  dragged  in  the  same  direction  with  a 
torque,  which,  up  to  a  certain  point,  would  go  on  increasing  with 
the  speed  of  rotation,  then  fall  off,  and,  passing  through  zero, 
finally  become  negative ;  that  is,  the  field  would  begin  to  rotate 
in  opposite  direction  to  the  disc.  In  experiments  with  alternate 
current  motors  in  which  the  field  was  shifted  by  currents  of 
differing  phase,  this  interesting  result  was  observed.  For  very 
low  speeds  of  rotation  of  the  field  the  motor  would  show  a 
torque  of  900  Ibs.  or  more,  measured  on  a  pulley  12  inches 
in  diameter.  When  the  speed  of  rotation  of  the  poles  was 
increased,  the  torque  would  diminish,  would  finally  go  down  to 
zero,  become  negative,  and  then  the  armature  would  begin  to 
rotate  in  opposite  direction  to  the  field. 

To  return  to  the  principal  subject ;  assume  the  conditions  to  be 
such  that  the  eddy  currents  generated  by  the  rotation  of  the  disc 
strengthen  the  field,  and  suppose  the  latter  gradually  removed 
while  the  disc  is  kept  rotating  at  an  increased  rate.  The  current, 
once  started,  may  then  be  sufficient  to  maintain  itself  and  even 
increase  in  strength,  and  then  we  have  the  case  of  Sir  William 
Thomson's  "current  accumulator."  But  from  the  above  con- 
siderations it  would  seem  that  for  the  success  of  the  experi- 
ment the  employment  of  a  disc  not  subdivided1  would  be  es- 
sential, for  if  there  should  be  a  radial  subdivision,  the  eddy  cur- 
rents could  not  form  and  the  self -exciting  action  would  cease.  If 
such  a  radially  subdivided  disc  were  used  it  would  be  necessary 
to  connect  the  spokes  by  a  conducting  rim  or  in  any  proper 
manner  so  as  to  form  a  symmetrical  system  of  closed  circuits. 

The  action  of  the  eddy  currents  may  be  utilized  to  excite  a  ma- 
chine of  any  construction.  For  instance,  in  Figs.  293  and  294  an 
arrangement  is  shown  by  which  a  machine  with  a  disc  armature 
might  be  excited.  Here  a  number  of  magnets,  N  s,  N  s,  are 
placed  radially  on  each  side  of  a  metal  disc  D  carrying  on  its  rim 
a  set  of  insulated  coils,  c  c.  The  magnets  form  two  separate 
fields,  an  internal  and  an  external  one,  the  solid  disc  rotating  in  the 

1.  Mr.  Tesla  here  refers  to  an  interesting  article  which  appeared  in  July, 
1865,  in  the  Phil.  Magazine,  by  Sir  W.  Thomson,  in  which  Sir  William, 
speaking  of  his  "  uniform  electric  current  accumulator,"  assumes  that  for 
self-excitation  it  is  desirable  to  subdivide  the  disc  into  an  infinite  number  of  in- 
finitely thin  spokes,  in  order  to  prevent  diffusion  of  the  current.  Mr.  Tesla 
shows  that  diffusion  is  absolutely  necessary  for  the  excitation  and  that  when 
the  disc  is  subdivided  no  excitation  can  occur. 


472  INVENTIONS  OF  NIKOLA  TESLA. 

field  nearest  the  axis,  and  the  coils  in  the  field  further  from  it. 
Assume  the  magnets  slightly  energized  at  the  start ;  they  could  be 
strengthened  by  the  action  of  the  eddy  currents  in  the  solid  disc 
so  as  to  afford  a  stronger  field  for  the  peripheral  coils.  Although 
there  is  no  doubt  that  under  proper  conditions  a  machine  might 
be  excited  in  this  or  a  similar  manner,  there  being  sufficient  ex- 
perimental evidence  to  warrant  such  an  assertion,  such  a  mode  of 
excitation  would  be  wasteful. 

But  a  unipolar  dynamo  or  motor,  such  as  shown  in  Fig.  292, 
may  be  excited  in  an  efficient  manner  by  simply  properly  subdi- 
viding the  disc  or  cylinder  in  which  the  currents  are  set  up,  and 
it  is  practicable  to  do  away  with  the  field  coils  which  are  usually 
employed.  Such  a  plan  is  illustrated  in  Fig.  295.  The  disc  or 


FIG.  293.  FIG.  294. 

cylinder  D  is  supposed  to  be  arranged  to  rotate  between  the  two 
poles  N  and  s  of  a'  magnet,  which  completely  cover  it  on  both 
sides,  the  contours  of  the  disc  and  poles  being  represented  by  the 
circles  d  and  d1  respectively,  the  upper  pole  being  omitted  for 
the  sake  of  clearness.  The  cores  of  the  magnet  are  supposed  to 
be  hollow,  the  shaft  c  of  the  disc  passing  through  them.  If  the 
unmarked  pole  be  below,  and  the  disc  be  rotated  screw  fashion, 
the  current  will  be,  as  before,  from  the  centre  to  the  periphery, 
and  may  be  taken  off  by  suitable  sliding  contacts,  B  B',  on  the 
shaft  and  periphery  respectively.  In  this  arrangement  the  cur- 
rent flowing  through  the  disc  and  external  circuit  will  have  no 
appreciable  effect  on  the  field  magnet. 

But  let  us  now  suppose  the  disc  to  be  subdivided  spirally,  as 


UNIPOLAR  GENERATORS. 


478 


indicated  by  the  full  or  dotted  lines,  Fig.  295.  The  difference  of 
potential  between  a  point  on  the  shaft  and  a  point  on  the  peri- 
phery will  remain  unchanged,  in  sign  as  well  as  in  amount.  The 
only  difference  will  be  that  the  resistance  of  the  disc  will  be  aug- 
mented and  that  there  will  be  a  greater  fall  of  potential  from  a 
point  on  the  shaft  to  a  point  on  the  periphery  when  the  same  cur- 
rent is  traversing  the  external  circuit.  But  since  the  current  is 
forced  to  follow  the  lines  of  subdivision,  we  see  that  it  will  tend 
either  to  energize  or  de-energize  the  field,  and  this  will  depend, 
other  things  being  equal,  upon  the  direction  of  the  lines  of  sub- 
division. If  the  subdivision  be  as  indicated  by  the  full  lines  in 
Fig.  295,  it  is  evident  that  if  the  current  is  of  the  same  direction 
as  before,  that  is,  from  centre  to  periphery,  its  effect  will  be  to 
strengthen  the  field  magnet;  whereas,  if  the  subdivision  be  as  in- 


FIG.  295. 


FIG.  296. 


dicated  by  the  dotted  lines,  the  current  generated  will  tend  to 
weaken  the  magnet.  In  the  former  case  the  machine  will  be 
capable  of  exciting  itself  when  the  disc  is  rotated  in  the  direction 
of  arrow  D  ;  in  the  latter  case  the  direction  of  rotation  must  be 
reversed.  Two  such  discs  may  be  combined,  however,  as  indi- 
cated, the  two  discs  rotating  in  opposite  fields,  and  in  the  same 
or  opposite  direction. 

Similar  disposition  may,  of  course,  be  made  in  a  type  of 
machine  in  which,  instead  of  a  disc,  a  cylinder  is  rotated.  In 
such  unipolar  machines,  in  the  manner  indicated,  the  usual  field 
coils  and  poles  may  be  omitted  and  the  machine  may  be  made  to 
consist  only  of  a  cylinder  or  of  two  discs  enveloped  by  a  metal 
casting. 

Instead  of  subdividing  the  disc  or  cylinder  spirally,  as  indicated 
in  Fig.  295,  it  is  more  convenient  to  interpose  one  or  more  turns 


474  INVENTIONS  OF  NIKOLA  TESLA. 

between  the  disc  and  the  contact  ring  on  the  periphery,  as  illus- 
trated in  Fig.  296. 

A  Forbes  dynamo  may,  for  instance,  be  excited  in  such  a  man- 
ner. In  the  experience  of  the  writer  it  has  been  found  that  in- 
stead of  taking  the  current  from  two  such  discs  by  sliding 
contacts,  as  usual,  a  flexible  conducting  belt  may  be  employed 
to  advantage.  The  discs  are  in  such  case  provided  with  large 
flanges,  affording  a  very  great  contact  surface.  The  belt  should 
be  made  to  bear  on  the  flanges  with  spring  pressure  to  take  up 
the  expansion.  Several  machines  with  belt  contact  were  con- 
structed by  the  writer  two  years  ago,  and  worked  satisfactorily  ; 
but  for  want  of  time  the  work  in  that  direction  has  been  tempor- 
arily suspended.  A  number  of  features  pointed  out  above  have 
also  been  used  by  the  writer  in  connection  with  some  types  of 
alternating  current  motors. 


PART  IV. 


APPENDIX.-EARLY  PHASE  MOTORS  AND  THE 
TESLA  MECHANICAL  AND  ELEC- 
TRICAL OSCILLATOR. 


CHAPTER   XLII. 

MR.  TESLA'S  PERSONAL  EXHIBIT  AT  THE  WORLD'S  FAIR. 

WHILE  the  exhibits  of  firms  engaged  in  the  manufacture  of 
electrical  apparatus  of  every  description  at  the  Chicago  World's 
Fair,  afforded  the  visitor  ample  opportunity  for  gaining  an  ex- 
cellent knowledge  of  the  state  of  the  art,  there  were  also  numbers 
of  exhibits  which  brought  out  in  strong  relief  the  work  of  the 
individual  inventor,  which  lies  at  the  foundation  of  much,  if  not 
all,  industrial  or  mechanical  achievement.  Prominent  among 
such  personal  exhibits  was  that  of  Mr.  Tesla,  whose  apparatus 
occupied  part  of  the  space  of  the  Westinghouse  Company,  in 
Electricity  Building. 

This  apparatus  represented  the  results  of  work  and  thought 
covering  a  period  of  ten  years.  It  embraced  a  large  number  of 
different  alternating  motors  and  Mr.  Tesla's  earlier  high  fre- 
quency apparatus.  The  motor  exhibit  consisted  of  a  variety  of 
fields  and  armatures  for  two,  three  and  multiphase  circuits,  and 
gave  a  fair  idea  of  the  gradual  evolution  of  the  fundamental  idea 
of  the  rotating  magnetic  field.  The  high  frequency  exhibit  in- 
cluded Mr.  Tesla's  earlier  machines  and  disruptive  discharge  coils 
and  high  frequency  transformers,  which  he  used  in  his  investi- 
gations and  some  of  which  are  referred  to  in  his  papers  printed 
in  this  volume. 

Fig.  297  shows  a  view  of  part  of  the  exhibits  containing  the 
motor  apparatus.  Among  these  is  shown  at  A  a  large  ring  in- 
tended to  exhibit  the  phenomena  of  the  rotating  magnetic  field. 
The  field  produced  was  very  powerful  and  exhibited  striking 
effects,  revolving  copper  balls  and  eggs  and  bodies  of  various 
shapes  at  considerable  distances  and  at  great  speeds.  This  ring 
was  wound  for  two-phase  circuits,  and  the  winding  was  so  dis- 
tributed that  a  practically  uniform  field  was  obtained.  This  ring 
was  prepared  for  Mr.  Tesla's  exhibit  by  Mr.  C.  F.  Scott,  elec- 
trician of  the  Westinghouse  Electric  and  Manufacturing  Com- 
pany. 


478 


INVENTION*  OF  NIKOLA  7/:s/..|. 


PERSONAL  EXHIBIT  AT  THE  WORLD? 8  FAIR.  479 

A  smaller  ring,  shown  at  B,  was  arranged  like  the  one  exhibited 
at  A  but  designed  especially  to  exhibit  the  rotation  of  an 
armature  in  a  rotating  field.  In  connection  with  these  two 
rings  there  was  an  interesting  exhibit  shown  by  Mr.  Tesla  which 
consisted  of  a  magnet  with  a  coil,  the  magnet  being  arranged  to 
rotate  in  bearings.  With  this  magnet  he  first  demonstrated  the 
identity  between  a  rotating  field  and  a  rotating  magnet ;  the  latter, 
when  rotating,  exhibited  the  same  phenomena  as  the  rings  when 
they  were  energized  by  currents  of  differing  phase.  Another 
prominent  exhibit  was  a  model  illustrated  at  c  which  is  a  two- 
phase  motor,  as  well  as  an  induction  motor  and  transformer.  It 
consists  of  a  large  outer  ring  of  laminated  iron  wound  with 
two  superimposed,  separated  windings  which  can  be  connected 
in  a  variety  of  ways.  This  is  one  of  the  first  models  used  by 
Mr.  Tesla  as  an  induction  motor  and  rotating  transformer.  The 
armature  was  either  a  steel  or  wrought  iron  disc  with  a  closed 
coil.  When  the  motor  was  operated  from  a  two  phase  generator 
the  windings  were  connected  in  two  groups,  as  usual.  When 
used  as  an  induction  motor,  the  current  induced  in  one  of  the 
windings  of  the  ring  was  passed  through  the  other  winding  on 
the  ring  and  so  the  motor  operated  with  only  two  wires.  When 
iised  as  a  transformer  the  outer  winding  served,  for  instance,  as 
a  secondary  and  the  inner  as  a  primary.  The  model  shown  at 
D  is  one  of  the  earliest  rotating  field  motors,  consisting  of  a  thin 
iron  ring  wound  with  two  sets  of  coils  and  an  armature  consisting 
of  a  series  of  steel  discs  partly  cut  away  and  arranged  on  a  small 
arbor. 

At  E  is  shown  one  of  the  first  rotating  field  or  induction  motors 
used  for  the  regulation  of  an  arc  lamp  and  for  other  purposes.  It 
comprises  a  ring  of  discs  with  two  sets  of  coils  having  different 
self-inductions,  one  set  being  of  German  silver  and  the  other  of 
copper  wire.  The  armature  is  wound  with  two  closed-circuited 
coils  at  right  angles  to  each  other.  To  the  armature  shaft  are 
fastened  levers  and  other  devices  to  effect  the  regulation.  At  F 
is  shown  a  model  of  a  magnetic  lag  motor ;  this  embodies  a  cast- 
ing with  pole  projections  protruding  from  two  coils  between 
which  is  arranged  to  rotate  a  smooth  iron  body.  When  an  alter- 
nating current  is  sent  through  the  two  coils  the  pole  projections 
of  the  field  and  armature  within  it  are  similarly  magnetized,  and 
upon  the  cessation  or  reversal  of  the  current  the  armature  and 
field  repel  each  other  and  rotation  is  produced  in  this  way. 


480  INVENTIONS  OF  NIKOLA  TE SLA. 

Another  interesting  exhibit,  shown  at  G,  is  an  early  model  of  a 
two  field  motor  energized  by  currents  of  different  phase.  There 
are  two  independent  fields  of  laminated  iron  joined  by  brass' 
bolts ;  in  each  field  is  mounted  an  armature,  both  armatures  be- 
ing on  the  same  shaft.  The  armatures  were  originally  so  ar- 
ranged as  to  be  placed  in  any  position  relatively  to  each  other, 
and  the  fields  also  were  arranged  to  be  connected  in  a  number 
of  ways.  The  motor  has  served  for  the  exhibition  of  a  number 
of  features ;  among  other  things,  it  has  been  used  as  a  dynamo 
for  the  production  of  currents  of  any  frequency  between  wide 
limits.  In  this  case  the  field,  instead  of  being  energized  by  di- 
rect current,  was  energized  by  currents  differing  in  phase,  which 


FIG.  298. 

produced  a  rotation  of  the  field  ;  the  armature  was  then  rotated 
in  the  same  or  in  opposite  direction  to  the  movement  of  the  field; 
and  so  any  number  of  alternations  of  the  currents  induced  in  the 
armature,  from  a  small  to  a  high  number,  determined  by  the 
frequency  of  the  energizing  field  coils  and  the  speed  of  the  arma- 
ture, was  obtained. 

The  models  H,  i,  j,  represent  a  variety  of  rotating  field,  synchron- 
ous motors  which  are  of  special  value  in  long  distance  transmis- 
sion work.  The  principle  embodied  in  these  motors  was  enunci- 
ated by  Mr.  Tesla  in  his  lecture  before  the  American  Institute  of 
Electrical  Engineers,  in  May,  18881.  It  involves  the  production 

1.  See  Part  I,  Chap.  Ill,  page  9. 


PERSONAL  EXHIBIT  AT  TllK  WORLD'S  FAIR.  481 

of  the  rotating  field  in  one  of  the  elements  of  the  motor  by  cur 
rents  differing  in  phase  and  energizing  the  other  element  by 
direct  currents.  The  armatures  are  of  the  two  and  three  phase 
type.  K  is  a  model  of  a  motor  shown  in  an  enlarged  view  in  Fig. 
298.  This  machine,  together  with  that  shown  in  Fig.  299,  was 
exhibited  at  the  same  lecture,  in  May,  1888.  They  were 
the  first  rotating  field  motors  which  were  independently  tested, 
having  for  that  purpose  been  placed  in  the  hands  of  Prof.  An- 
thony in  the  winter  of  188T-88.  From  these  tests  it  was  shown 
that  the  efficiency  and  output  of  these  motors  was  quite  satisfac- 
tory in  every  respect. 

It  was  intended  to  exhibit  the  model  shown  in  Fig.  299,  but  it 
was  unavailable  for  that  purpose  owing  to  the  fact  that  it  was 


FIG.  299. 

some  time  ago  handed  over  to  the  care  of  Prof.  Ayrton  in  Eng- 
land. This  model  was  originally  provided  with  twelve  independ- 
ent coils ;  this  number,  as  Mr.  Tesla  pointed  out  in  his  first  lec- 
ture, being  divisible  by  two  and  three,  was  selected  in  order  to  make 
various  connections  for  two  and  three-phase  operations,  and  during 
Mr.  Tesla's  experiments  was  used  in  many  ways  with  from  two  to 
six  phases.  The  model,  Fig.  298,  consists  of  a  magnetic  frame  of 
laminated  iron  with  four  polar  projections  between  which  an  arm- 
ature is  supported  on  brass  bolts  passing  through  the  frame.  A 
great  variety  of  armatures  was  used  in  connection  with  these  two 
and  other  fields.  Some  of  the  armatures  are  shown  in  front  on 
the  table,  Fig.  297,  and  several  are  also  shown  enlarged  in  Figs. 
300  to  310.  An  interesting  exhibit  is  that  shown  at  L,  Fig.  297.. 
This  is  an  armature  of  hardened  steel  which  was  used  in  a  demon- 


482 


INVENTIONS  OF  NIKOLA  TESLA. 


stration  before  the  Society  of  Arts  in  Boston,  by  Prof.  Anthony. 
Another  curious  exhibit  is  shown  enlarged  in  Fig.  301.  This 
consists  of  thick  discs  of  wrought  iron  placed  lengthwise,  with  a- 
mass  of  copper  cast  around  them.  The  discs  were  arranged 
longitudinally  to  afford  an  easier  starting  by  reason  of  the  induced 
current  formed  in  the  iron  discs,  which  differed  in  phase  from 
those  in  the  copper.  This  armature  would  start  with  a  single  cir- 
cuit and  run  in  synchronism,  and  represents  one  of  the  earliest 
types  of  such  an  armature.  Fig.  305  is  another  striking  exhibit. 


FIG.  303. 


FIG.  304. 


FIG.  305. 


FIG.  306 


FIG.  307 


FIG.  308. 


FIG.  309. 


FIG.  310. 


This  is  one  of  the  earliest  types  of  an  armature  with  holes  beneath 
the  periphery,  in  which  copper  conductors  are  imbedded.  The 
armature  has  eight  closed  circuits  and  was  used  in  many  different 
ways.  Fig.  304  is  a  type  of  synchronous  armature  consisting  of 
a  block  of  soft  steel  wound  with  a  coil  closed  upon  itself.  This 
armature  was  used  in  connection  with  the  field  shown  in  Fig.  298 
and  gave  excellent  results. 

Fig.  302  represents  a  synchronous  armature  with  a  large  coil 
around  a  body  of  iron.  There  is  another  very  small  coil  at  right 
angles  to  the  first.  This  small  coil  was  used  for  the  purpose  of 


PERSONAL  EXHIBIT  AT  THE  WORLD'S  FAIR.  483 

increasing  the  starting  torque  and  was  found  very  effective  in 
this  connection.  Figs.  306  and  308  show  a  favorite  construction 
of  armature ;  the  iron  body  is  made  up  of  two  sets  of  discs  cut 
away  and  placed  at  right  angles  to  each  other,  the  interstices  be- 
ing wound  with  coils.  The  one  shown  in  Fig.  308  is  provided 
with  an  additional  groove  on  each  of  the  projections  formed  by 
the  discs,  for  the  purpose  of  increasing  the  starting  torque  by  a 
wire  wound  in  these  projections.  Fig.  307  is  a  form  of  armature 
similarly  constructed,  but  with  four  independent  coils  wound  upon 
the  four  projections.  This  armature  was  used  to  reduce  the 
speed  of  the  motor  with  reference  to  that  of  the  generator.  Fig. 
300  is  still  another  armature  with  a  great  number  of  independent 
circuits  closed  upon  themselves,  so  that  all  the  dead  points  on 
the  armature  are  done  away  with,  and  the  armature  has  a  large 
starting  torque.  Fig.  303  is  another  type  of  armature  for  a  four- 
pole  motor  but  with  coils  wound  upon  a  smooth  surface.  A 
number  of  these  armatures  have  hollow  shafts,  as  they  have  been 
used  in  many  ways.  Figs.  309  and  310  represent  armatures  to 
which  either  alternating  or  direct  current  was  conveyed  by 
means  of  sliding  rings.  Fig.  309  consists  of  a  soft  iron  body 
with  a  single  coil  wound  around  it,  the  ends  of  the  coil  being 
connected  to  two  sliding  rings  to  which,  usually,  direct  current 
was  conveyed.  The  armature  shown  in  Fig.  310  has  three  insu- 
lated rings  on  a  shaft  and  was  used  in  connection  with  two  or 
three  phase  circuits. 

All  these  models  shown  represent  early  work,  and  the  en- 
larged engravings  are  made  from  photographs  taken  early  in 
1888.  There  is  a  great  number  of  other  models  which  were  ex- 
hibited, but  which  are  not  brought  out  sharply  in  the  engraving, 
Fig.  297.  For  example  at  M  is  a  model  of  a  motor  comprising 
an  armature  with  a  hollow  shaft  wound  with  two  or  three  coils  for 
two  or  three-phase  circuits ;  the  armature  was  arranged  to  be  sta- 
tionary and  the  generating  circuits  were  connected  directly  to 
the  generator.  Around  the  armature  is  arranged  to  rotate  on 
its  shaft  a  casting  forming  six  closed  circuits.  On  the  outside 
this  casting  was  turned  smooth  and  the  belt  was  placed  on  it  for 
driving  with  any  desired  appliance.  This  also  is  a  very  early 
model. 

On  the  left  side  of  the  table  there  are  seen  a  large  variety  of 
models,  N,  o,  P,  etc.,  with  fields  of  various  shapes.  Each  of  these 
models  involves  some  distinct  idea  and  they  all  represent  gradual 


484  INVENTIONS  OF  NIKOLA  TESLA. 

development  chiefly  interesting  as  showing  Mr.  Tesla's  efforts  to 
adapt  his  system  to  the  existing  high  frequencies. 

On  the  right  side  of  the  table,  at  s,  T,  are  shown,  on  separate 
supports,  larger  and  more  perfected  armatures  of  commercial 
motors,  and  in  the  space  around  the  table  a  variety  of  motors  and 
generators  supplying  currents  to  them  was  exhibited. 

The  high  frequency  exhibit  embraced  Mr.  Tesla's  first  original 
apparatus  used  in  his  investigations.  There  was  exhibited  a 
glass  tube  with  one  layer  of  silk-covered  wire  wound  at  the  top 
and  a  copper  ribbon  on  the  inside.  This  was  the  first  disruptive 
discharge  coil  constructed  by  him.  At  u  is  shown  the  disruptive 


FIG.  811. 

discharge  coil  exhibited  by  him  in  his  lecture  before  the  Ameri- 
can Institute  of  Electrical  Engineers,  in  May,  1891.1  At  v  and  w 
are  shown  some  of  the  first  high  frequency  transformers.  A 
number  of  various  fields  and  armatures  of  small  models  of  high 
frequency  apparatus  as  shown  at  x  and  Y,  and  others  not  visible 
in  the  picture,  were  exhibited.  In  the  annexed  space  the  dynamo 
then  used  by  Mr.  Tesla  at  Columbia  College  was  exhibited  ;  also 
another  form  of  high  frequency  dynamo  used. 

In  this  space  also  was  arranged  a  battery  of  Leyden  jars  and 
his  large  disruptive  discharge  coil  which  was  used  for  exhibiting 

1.    See  Part  II,  Chap.  XXVI.,  page  145. 


PERSONAL  EXHIBIT  AT  THE  WORLD' 8  FAIR.  485 

the  light  phenomena  in  the  adjoining  dark  room.  The  coil  was 
operated  at  only  a  small  fraction  of  its  capacity,  as  the  necessary 
condensers  and  transformers  could  not  be  had  and  as  Mr.  Tesla's 
stay  was  limited  to  one  week ;  notwithstanding,  the  phenomena 
were  of  a  striking  character.  In  the  room  were  arranged  two 
large  plates  placed  at  a  distance  of  about  eighteen  feet  from  each 
other.  Between  them  were  placed  two  long  tables  with  all  sorts 
of  phosphorescent  bulbs  and  tubes ;  many  of  these  were  prepared 
with  great  care  and  marked  legibly  with  the  names  which  would 
shine  with  phosphorescent  glow.  Among  them  were  some  with 
the  names  of  Helmholtz,  Faraday,  Maxwell,  Henry,  Franklin) 
etc.  Mr.  Tesla  had  also  not  forgotten  the  greatest  living  poet  of 
his  own  country,  Zmaj  Jovan ;  two  or  three  were  prepared  with 
inscriptions,  like  "  Welcome,  Electricians,"  and  produced  a  beau- 
tiful effect.  Each  represented  some  phase  of  this  work  and  stood 
for  some  individual  experiment  of  importance.  Outside  the  room 
was  the  small  battery  seen  in  Fig.  311,  for  the  exhibition  of  some 
of  the  impedance  and  other  phenomena  of  interest.  Thus,  for 
instance,  a  thick  copper  bar  bent  in  arched  form  was  provided 
with  clamps  for  the  attachment  of  lamps,  and  a  number  of  lamps 
were  kept  at  incandescence  on  the  bar ;  there  was  also  a  little  mo- 
tor shown  on  the  table  operated  by  the  disruptive  discharge. 

As  will  be  remembered  by  those  who  visited  the  Exposition, 
the  Westinghouse  Company  made  a  fine  exhibit  of  the  various 
commercial  motors  of  the  Tesla  system,  while  the  twelve  genera- 
tors in  Machinery  Hall  were  of  the  two-phase  type  constructed 
for  distributing  light  and  power.  Mr.  Tesla,  also  exhibited 
some  models  of  his  oscillators. 


CHAPTER   XLIII. 

THE  TESLA  MECHANICAL  AND  ELECTRICAL  OSCILLATORS. 

ON  the  evening  of  Friday,  August  25,  1893,  Mr.  Tesla  de- 
livered a  lecture  on  his  mechanical  and  electrical  oscillators,  be- 
fore the  members  of  the  Electrical  Congress,  in  the  hall  adjoin- 
ing the  Agricultural  Building,  at  the  World's  Fair,  Chicago.  Ber 
sides  the  apparatus  in  the  room,  he  employed  an  air  compressor, 
which  was  driven  by  an  electric  motor. 

Mr.  Tesla  was  introduced  by  Dr.  Elisha  Gray,  and  began  by 
stating  that  the  problem  he  had  set  out  to  solve  was  to  construct, 
first,  a  mechanism  which  would  produce  oscillations  of  a  per- 
fectly constant  period  independent  of  the  pressure  of  steam  or 
air  applied,  within  the  widest  limits,  and  also  independent  of 
frictional  losses  and  load.  Secondly,  to  produce  electric  cur- 
rents of  a  perfectly  constant  period  independently  of  the  work- 
ing conditions,  and  to  produce  these  currents  with  mechanism 
which  should  be  reliable  and  positive  in  its  action  without  resort- 
ing to  spark  gaps  and  breaks.  This  he  successfully  accomplished 
in  his  apparatus,  and  with  this  apparatus,  now,  scientific  men  will 
be  provided  with  the  necessaries  for  carrying  on  investigations 
with  alternating  currents  with  great  precision.  These  two  in- 
ventions Mr.  Tesla  called,  quite  appropriately,  a  mechanical  and 
an  electrical  oscillator,  respectively. 

The  former  is  substantially  constructed  in  the  following  way. 
There  is  a  piston  in  a  cylinder  made  to  reciprocate  automatically 
by  proper  dispositions  of  parts,  similar  to  a  reciprocating  tool. 
Mr.  Tesla  pointed  out  that  he  had  done  a  great  deal  of  work  in 
perfecting  his  apparatus  so  that  it  would  work  efficiently  at  such 
high  frequency  of  reciprocation  as  he  contemplated,  but  he  did  not 
dwell  on  the  many  difficulties  encountered.  He  exhibited,  how- 
ever, the  pieces  of  a  steel  arbor  which  had  been  actually  torn 
apart  while  vibrating  against  a  minute  air  cushion. 

With  the  piston  above  referred  to  there  is  associated  in  one  of 
his  models  in  an  independent  chamber  an  air  spring,  or  dash  pot, 


MECUANICAL  AND  ELECTRICAL  OSCILLATORS.  487 

or  else  he  obtains  the  spring  within  the  chambers  of  the  oscillator 
itself.  To  appreciate  the  beauty  of  this  it  is  only  necessary  to  say 
that  in  that  disposition,  as  he  showed  it,  no  matter  what  the 
rigidity  of  the  spring  and  no  matter  what  the  weight  of  the  mov- 
ing parts,  in  other  words,  no  matter  what  the  period  of  vibrations, 
the  vibrations  of  the  spring  are  always  isochronous  with  the  ap- 
plied pressure.  Owing  to  this,  the  results  obtained  with  these 
vibrations  are  truly  wonderful.  Mr.  Tesla  provides  for  an  air 
spring  of  tremendous  rigidity,  and  he  is  enabled  to  vibrate  big 
weights  at  an  enormous  rate,  considering  the  inertia,  owing  to  the 
recoil  of  the  spring.  Thus,  for  instance,  in  one  of  these  experi- 
ments, he  vibrates  a  weight  of  approximately  20  pounds  at  the 
rate  of  about  80  per  second  and  with  a  stroke  of  about  f-  inch,  but 
by  shortening  the  stroke  the  weight  could  be  vibrated  many  hun- 
dred times,  and  has  been,  in  other  experiments. 

To  start  the  vibrations,  a  powerful  blow  is  struck,  but  the  ad- 
justment can  be  so  made  that  only  a  minute  effort  is  required  to 
start,  and,  even  without  any  special  provision  it  will  start  by 
merely  turning  on  the  pressure  suddenly.  The  vibration  being, 
of  course,  isochronous,  any  change  of  pressure  merely  produces  a 
shortening  or  lengthening  of  the  stroke.  Mr.  Tesla  showed  a 
number  of  very  clear  drawings,  illustrating  the  construction  of 
the  apparatus  from  which  its  working  was  plainly  discernible. 
Special  provisions  are  made  so  as  to  equalize  the  pressure 
within  the  dash  pot  and  the  outer  atmosphere.  For  this  purpose 
the  inside  chambers  of  the  dash  pot  are  arranged  to  communi- 
cate with  the  outer  atmosphere  so  that  no  matter  how  the  tempera- 
ture of  the  enclosed  air  might  vary,  it  still  retains  the  same  mean 
density  as  the  outer  atmosphere,  and  by  this  means  a  spring  of 
constant  rigidity  is  obtained.  Now,  of  course,  the  pressure  of 
the  atmosphere  may  vary,  and  this  would  vary  the  rigidity  of  the 
spring,  and  consequently  the  period  of  vibration,  and  this  feature 
constitutes  one  of  the  great  beauties  of  the  apparatus ;  for,  as  Mr. 
Tesla  pointed  out,  this  mechanical  system  acts  exactly  like  a 
string  tightly  stretched  between  two  points,  and  with  fixed  nodes, 
so  that  slight  changes  of  the  tension  do  not  in  the  least  alter  the 
period  of  oscillation. 

The  applications  of  such  an  apparatus  are,  of  course,  numer- 
ous and  obvious.  The  first  is,  of  course,  to  produce  electric 
currents,  and  by  a  number  of  models  and  apparatus  on  the  lecture 
platform,  Mr.  Tesla  showed  how  this  could  be  carried  out  in 


488  INVENTIONS  OF  NIKOLA  TESLA. 

practice  by  combining  an  electric  generator  with  his  oscillator. 
He  pointed  out  what  conditions  must  be  observed  in  order  that 
the  period  of  vibration  of  •the  electrical  system  might  not  disturb- 
the  mechanical  oscillation  in  such  a  way  as  to  alter  the  periodicity, 
but  merely  to  shorten  the  stroke.  He  combines  a  condenser 
with  a  self-induction,  and  gives  to  the  electrical  system  the  same 
period  as  that  at  which  the  machine  itself  oscillates,  so  that  both 
together  then  fall  in  step  and  electrical  and  mechanical  resonance 
is  obtained,  and  maintained  absolutely  unvaried. 

IsText  he  showed  a  model  of  a  motor  with  delicate  wheelwork, 
which  was  driven  by  these  currents  at  a  constant  speed,  no  mat- 
ter what  the  air  pressure  applied  was,  so  that  this  motor  could 
be  employed  as  a  clock.  He  also  showed  a  clock  so  constructed 
that  it  could  be  attached  to  one  of  the  oscillators,  and  would 
keep  absolutely  correct  time.  Another  curious  and  interesting 
feature  which  Mr.  Tesla  pointed  out  was  that,  instead  of  con- 
trolling the  motion  of  the  reciprocating  piston  by  means  of  a 
spring,  so  as  to  obtain  isochronous  vibration,  he  was  actually  able 
to  control  the  mechanical  motion  by  the  natural  vibration  of  the 
electro-magnetic  system,  and  he  said  that  the  case  was  a  very 
simple  one,  and  was  quite  analogous  to  that  of  a  pendulum. 
Thus,  supposing  we  had  a  pendulum  of  great  weight,  preferably, 
which  would  be  .maintained  in  vibration  by  force,  periodically 
applied  ;  now  that  force,  no  matter  how  it  might  vary,  although 
it  would  oscillate  the  pendulum,  would  have  no  control  over  its 
period. 

Mr.  Tesla  also  described  a  very  interesting  phenomenon  which 
he  illustrated  by  an  experiment.  By  means  of  this  new  appara- 
tus, he  is  able  to  produce  an  alternating  current  in  which  the 
E.  M.  F.  of  the  impulses  in  one  direction  preponderates  over  that 
of  those  in  the  other,  so  that  there  is  produced  the  effect  of  a 
direct  current.  In  fact  he  expressed  the  hope  that  these  cur- 
rents would  be  capable  of  application  in  many  instances,  serving 
as  direct  currents.  The  principle  involved  in  this  preponderat- 
ing E.  M.  F.  he  explains  in  this  way :  Suppose  a  conductor  is 
moved  into  the  magnetic  field  and  then  suddenly  withdrawn.  If 
the  current  is  not  retarded,  then  the  work  performed  will  be  a 
mere  fractional  one ;  but  if  the  current  is  retarded,  then  the 
magnetic  field  acts  as  a  spring.  Imagine  that  the  motion  of  the 
conductor  is  arrested  by  the  current  generated,  and  that  at  the 
instant  when  it  stops  to  move  into  the  field,  there  is  still  the 


MECHANICAL  AND  ELECTRICAL  OSCILLATORS.  489 

maximum  current  flowing  in  the  conductor ;  then  this  current 
will,  according  to  Lenz's  law,  drive  the  conductor  out  of  the  field 
again,  and  if  the  conductor  has  no  resistance,  then  it  would  leave 
the  field  with  the  velocity  it  entered  it.  Now  it  is  clear  that  if, 
instead  of  simply  depending  on  the  current  to  drive  the  conduc- 
tor out  of  the  field,  the  mechanically  applied  force  is  so  timed 
that  it  helps  the  conductor  to  get  out  of  the  field,  then  it  might 
leave  the  field  with  higher  velocity  than  it  entered  it,  and 
thus  one  impulse  is  made  to  preponderate  in  E.  M.  r.  over  the 
other. 

With  a  current  of  this  nature,  Mr.  Tesla  energized  magnets 
strongly,  and  performed  many  interesting  experiments  bearing 
out  the  fact  that  one  of  the  current  impulses  preponderates. 
Among  them  was  one  in  which  he  attached  to  his  oscillator  a  ring 
magnet  with  a  small  air  gap  between  the  poles.  This  magnet  was 
oscillated  up  and  down  80  times  a  second.  A  copper  disc,  when 
inserted  within  the  air  gap  of  the  ring  magnet,  was  brought  into 
rapid  rotation.  Mr.  Tesla  remarked  that  this  experiment  also 
seemed  to  demonstrate  that  the  lines  of  flow  of  current  through 
a  metallic  mass  are  disturbed  by  the  presence  of  a  magnet  in  a 
manner  quite  independently  of  the  so-called  Hall  effect.  He 
showed  also  a  very  interesting  method  of  making  a  connection 
with  the  oscillating  magnet.  This  was  accomplished  by  attaching 
to  the  magnet  small  insulated  steel  rods,  and  connecting  to  these 
rods  the  ends  of  the  energizing  coil.  As  the  magnet  was  vibrated, 
stationary  nodes  were  produced  in  the  steel  rods,  and  at  these 
points  the  terminals  of  a  direct  current  source  were  attached. 
Mr.  Tesla  also  pointed  out  that  one  of  the  uses  of  currents,  such 
as  those  produced  in  his  apparatus,  would  be  to  select  any  given 
one  of  a  number  of  devices  connected  to  the  same  circuit  by  pick- 
ing out  the  vibration  by  resonance.  There  is  indeed  little  doubt 
that  with  Mr.  Tesla's  devices,  harmonic  and  synchronous  tele- 
graphy will  receive  a  fresh  impetus,  and  vast  possibilities  are 
again  opened  up. 

Mr.  Tesla  was  very  much  elated  over  his  latest  achievements, 
and  said  that  he  hoped  that  in  the  hands  of  practical,  as  well  as 
scientific  men,  the  devices  described  by  him  would  yield  important 
results.  He  laid  special  stress  on  the  facility  now  afforded  for 
investigating  the  effect  of  mechanical  vibration  in  all  directions, 
and  also  showed  that  he  had  observed  a  number  of  facts  in  con- 
nection with  iron  cores. 


490  INVENTIONS  OF  NIKOLA  TE8LA. 

The  engraving,  Fig.  312,  shows,  in  perspective,  one  of  the 
forms  of  apparatus  used  by  Mr.  Tesla  in  his  earlier  investigations 
in  this  field  of  work,  and  its  interior  construction  is  made  plain, 
by  the  sectional  view  shown  in  Fig.  313.  It  will  be  noted  that  the 
piston  P  is  fitted  into  the  hollow  of  a  cylinder  c  which  is  provided 
with  channel  ports  o  o,  and  i,  extending  all  around  the  inside 
surface.  In  this  particular  apparatus  there  are  two  channels  o  o 


for  the  outlet  of  the  working  fluid  and  one,  i,  for  the  inlet. 
The  piston  P  is  provided  with  two  slots  s  s'  at  a  carefully  deter- 
mined distance,  one  from  the  other.  The  tubes  T  T  which  are 
sere  wed  into  the  holes  drilled  into  the  piston,  establish  communi- 
cation between  the  slots  s  s'  and  chambers  on  each  side  of  the 
piston,  each  of  these  chambers  connecting  with  the  slot  which  is 
remote  from  it.  The  piston  P  is  screwed  tightly' on  a  shaft  A 


MECHANICAL  AND  ELECTRICAL  OSCILLATORS. 


491 


which  passes  through  fitting  boxes  at  the  end  of  the  cylinder  c. 
The  boxes  project  to  a  carefully  determined  distance  into  the  hol- 
low of  the  cylinder  c,  thus  determining  the  length  of  the  stroke. 

Surrounding  the  whole  is  a  jacket  j.  This  jacket  acts  chiefly  to 
diminish  the  sound  produced  by  the  oscillator  and  as  a  jacket  when 
the  oscillator  is  driven  by  steam,  in  which  case  a  somewhat  differ- 
ent arrangement  of  the  magnets  is  employed.  The  apparatus  here 
illustrated  was  intended  for  demonstration  purposes,  air  being 
used  as  most  convenient  for  this  purpose. 

A  magnetic  frame  .M  M  is  fastened  so  as  to  closely  surround  the 
oscillator  and  is  provided  with  energizing  coils  which  establish 


FIG.  313. 

two  strong  magnetic  fields  on  opposite  sides.  The  magnetic  frame 
is  made  up  of  thin  sheet  iron.  In  the  intensely  concentrated 
field  thus  produced,  there  are  arranged  two  pairs  of  coils  H  H  sup- 
ported in  metallic  frames  which  are  screwed  on  the  shaft  A  of 
the  piston  and  have  additional  bearings  in  the  boxes  B  B  on  each 
side.  The  whole  is  mounted  on  a  metallic  base  resting  on  two 
wooden  blocks. 

The  operation  of  the  device  is  as  follows :  The  working  fluid 
being  admitted  through  an  inlet  pipe  to  the  slot  i  and  the  piston 
being  supposed  to  be  in  the  position  indicated,  it  is  sufficient, 
though  not  necessary,  to  give  a  gentle  tap  on  one  of  the  shaft 


492  INVENTIONS  OF  NIKOLA  TE8LA. 

ends  protruding  from  the  boxes  B.  Assume  that  the  motion  im- 
parted be  such  as  to  move  the  piston  to  the  left  (when  looking  at 
the  diagram)  then  the  air  rushes  through  the  slot  s'  and  tube  T 
into  the  chamber  to  the  left.  The  pressure  now  drives  the  pie- 
ton  towards  the  right  and,  owing  to  its  inertia,  it  overshoots  the 
position  of  equilibrium  and  allows  the  air  to  rush  through  the 
slot  s  and  tube  T  into  the  chamber  to  the  right,  while  the  com- 
munication to  the  left  hand  chamber  is  cut  off,  the  air  of  the 
latter  chamber  escaping  through  the  outlet  o  on  the  left.  On 
the  return  stroke  a  similar  operation  takes  place  on  the  right 
hand  side.  This  oscillation  is  maintained  continuously  and  the 
apparatus  performs  vibrations  from  a  scarcely  perceptible  quiver 
amounting  to  no  more  than  l  of  an  inch,  up  to  vibrations  of  a  little 
over  |  of  an  inch,  according  to  the  air  pressure  and  load.  It  is 
indeed  interesting  to  see  how  an  incandescent  lamp  is  kept  burn- 
ing with  the  apparatus  showing  a  scarcely  perceptible  quiver. 

To  perfect  the  mechanical  part  of  the  apparatus  so  that  oscil- 
lations are  maintained  economically  was  one  thing,  and  Mr.  Tesla 
hinted  in  his  lecture  at  the  great  difficulties  he  had  first  encoun- 
tered to  accomplish  this.  But  to  produce  oscillations  which  would 
be  of  constant  period  was  another  task  of  no  mean  proportions. 
As  already  pointed  out,  Mr.  Tesla  obtains  the  constancy  of  period 
in  three  distinct  ways.  Thus,  he  provides  properly  calculated 
chambers,  as  in  the  case  illustrated,  in  the  oscillator  itself  ;  or  he  as- 
sociates with  the  oscillator  an  air  spring  of  constant  resilience.  But 
the  most  interesting  of  all,  perhaps,  is  the  maintenance  of  the  con- 
stancy of  oscillation  by  the  reaction  of  the  electromagnetic  part  of 
the  combination.  Mr.  Tesla  winds  his  coils,  by  preference,  for  high 
tension  and  associates  with  them  a  condenser,  making  the  natural 
period  of  the  combination  fairly  approximating  to  the  average  period 
at  which  the  piston  would  oscillate  without  any  particular  provision 
being  made  for  the  constancy  of  period  under  varying  pressure 
and  load.  As  the  piston  with  the  coils  is  perfectly  free  to  move, 
it  is  extremely  susceptible  to  the  influence  of  the  natural  vibra- 
tion set  up  in  the  circuits  of  the  coils  H  H.  The  mechanical  effici- 
ency of  the  apparatus  is  very  high  owing  to  the  fact  that  friction 
is  reduced  to  a  minimum  and  the  weights  which  are  moved  are 
small ;  the  output  of  the  oscillator  is  therefore  a  very  large  one. 

Theoretically  considered,  when  the  various  advantages  which 
Mr.  Tesla  holds  out  are  examined,  it  is  surprising,  considering 
the  simplicity  of  the  arrangement,  that  nothing  was  done  in  this 


MECHANICAL  AND  ELECTRICAL  OSCILLATORS.  493 

direction  before.  No  doubt  many  inventors,  at  one  time  or 
other,  have  entertained  the  idea  of  generating  currents  by  at- 
taching a  coil  or  a  magnetic  core  to  the  piston  of  a  steam  engine, 
or  generating  currents  by  the  vibrations  of  a  tuning  fork,  or 
similar  devices,  but  the  disadvantages  of  such  arrangements  from 
an  engineering  standpoint  must  be  obvious.  Mr.  Tesla,  however, 
in  the  introductory  remarks  of  his  lecture,  pointed  out  how  by  a 
series  of  conclusions  he  was  driven  to  take  up  this  new  line  of 
work  by  the  necessity  of  producing  currents  of  constant  period 
and  as  a  result  of  his  endeavors  to  maintain  electrical  oscillation 
in  the  most  simple  and  economical  manner. 


INDEX. 


Alternate  Current  Electrostatic 

Apparatus 392 

Alternating  Current  Generators 

for  High  Frequency..  152,  374,  224 
Alternating  Motors  and  Trans- 
formers         7 

American     Institute    Electrical 

Engineers  Lecture 145 

Anthony,  W.  A.,  Tests  of  Tesla 

Motors 8 

Apparatus  for  Producing  High 

Vacua 276 

Arc  Lighting,  Tesla  Direct,  Sys- 
tem  451 

Auxiliary  Brush  Regulation  ...  438 

Biography,  Tesla 4 

Brush.  Anti-Sparking 432 

"      Third,  Regulation 438 

Phenomena      in       High 

Vacuum 226 

Carborundum  Button  for  Tesla 

Lamps 140,  253 

Commutator,  Anti-Sparking. . . .  432 
Combination  of    Synchronizing 

and  Torque  Motor 94 

Condensers  with  Plates  in  Oil. .  418 
Conversion  with  Disruptive  Dis- 
charge  193,204,  303 

Current  or  Dynamic  Electricity 

Phenomena , . . .  327 

Direct  Current  Arc  Lighting 451 

Dischargers,  Forms  of 305 

Disruptive  Discharge  Coil . .  207,  221 
Disruptive  Discharge  Phenom- 
ena... ..  212 


Dynamos,  Improved  Direct  Cur- 
rent   448 

Early  Phase  Motors 477 

Effects  with    High    Frequency 

and  High  Potential  Currents.  119 
Electrical     Congress    Lecture, 

Chicago  486 

Electric  Resonance 340 

Electric  Discharges  in  Vacuum 

Tubes  396 

Electrolytic  Registering  Meter.  420 

Eye,  Observations  on  the 294 

Flames,  Electrostatic,  Non-Con- 
suming  166,272 

Forbes  Unipolar  Generator. 468,  474 

Franklin  Institute  Lecture 294 

Generators,  Pyromagnetic 429 

High  Potential,  High  Frequency  : 
Brush  Phenomena  in  High 

Vacuum 226 

Carborundum  Buttons.. 140,  253 
Disruptive  Discharge  Pheno- 
mena    212 

Flames,  Electrostatic,  Non- 
Consuming 166,  272 

Impedance,  Novel  Phenom- 
ena  194,  338 

Lighting   Lamps    Through 

Body 359 

Luminous      Effects      with 

Gases 368 

' '  Massage  "  with  Currents    394 
Motor  with  Single  Wire. 234,  330 

"No  Wire  "  Motors 235 

Oil  Insulation  of  Induction 
Coils 173,  221 


INDEX. 


495 


High  Potential.— Continued. 

Ozone,  Production  of 171 

Phosphorescence 367 

Physiological  Effects...  162,  394 

Resonance 340 

Spinning  Filament        .          168 
Streaming     Discharges    of 

High  Tension  Coil. .  ..155,  163 
Telegraphy  without  Wires    346 
Impedance,  Novel  Phenomena. 

194,  338 

Improvements  in  Unipolar  Gen- 
erators    465 

Improved  Direct  Current  Dyna- 
mos and  Motors 448 

Induction  Motors  92 

Institution  Electrical  Engineers 

Lecture 189 

Lamps  and  Motor  operated  on 

a  Single  Wire 330 

Lamps    with     Single     Straight 

Fiber 183 

Lamps  containing  only  a  Gas. .   188 
Lamps    with    Refractory    But- 
ton   177,  239,  360 

Lamps  for  Simple  Phosphore- 
scence...  187,  282,  364 

Lecture,  Tesla  before  : 

American  Institute  Elec- 
trical Engineers 145 

Royal  Institution  124 

Institution  Electrical  Eng- 
ineers   189 

Franklin  Institute  and  Nat- 
ional Electric  Light  Asso- 
ciation   294 

Electrical  Congress, Chicago  486 
Lighting  Lamps  Through  the 

Body   359 

Light    Phenomena    with   High 

Frequencies. 349 

Luminous  Effects  with  Gases  at 

Low  Pressure 368 

"  Magnetic  Lag  "  Motor 67 

"Massage"    with   Currents    of 

High  Frequency 394 

Mechanical  and  Electrical  Oscil- 
lators  486 

Method  of  obtaining  Direct  from 
Alternating  currents 409 


Method  of  obtaining  Difference 
of  Phase  by  Magnetic  Shield- 
ing   71 

Motors  : 

With  Circuits  of  Different 

Resistance 79 

With  Closed  Conductors. . .       9 
Combination  of  Synchroniz- 
ing and  Torque 94 

With  Condenser  in  Arma- 
ture Circuit 100 

With   Condenser  in  one  of 

the  Field  Circuits  106 

With  Coinciding  Maxima  of 
Magnetic  Effect  in  Arma- 
ture and  Field 83 

With  "Current  Lag  "  Arti- 
ficially Secured 58 

Early  Phase  477 

With  Equal  Magnetic  Ener- 
gies in  Field  and  Arma- 
ture  81 

Or  Generator,  obtaining  De- 
sired Speed  of 36 

Improved  Direct  Current. . .  448 

Induction 92 

' '  Magnetic  Lag  " 67 

"No  Wire" 235 

With  Phase  Difference    in 
Magnetization    of    Inner 
and  Outer  Parts  of  Core..     88 
Regulator  for  Rotary  Cur- 
rent      45 

Single  Circuit,  Self-starting 

Synchronizing ....     50 

Single  Phase 76 

With  Single  Wire  to  Genera- 
tor   234,  330 

Synchronizing 9 

Thermo-Magnetic    424 

Utilizing  Continuous  Cur- 
rent Generators . .  31 

National  Electric  Light  Asso- 
ciation Lecture 294 

"  No  Wire  "  Motor 285 

Observations  on  the  Eye. 294 

Oil,  Condensers  with  Plates  in  .  418 
Oil  Insulation  of  Induction  Coils 

173,  221 

Oscillators,  Mechanical  and 
Electrical...  ..  486 


INDEX. 


Ozone,  Production  of 171 

Phenomena  Produced  by  Elec- 
trostatic Force 319 

Phosphorescence  and  Sulphide 

of  Zinc 367 

Physiological    Effects  of    High 

Frequency 162,  394 

Polyphase  Systems 26 

Polyphase  Transformer 109 

Pyromagnetic  Generators 429 

Regulator   for  Rotary   Current 

Motors 45 

Resonance,    Electric,    Phenom- 
ena of 340 

' '  Resultant  Attraction  " 7 

Rotating  Field  Transformers. . .       9 

Rotating  Magnetic  Field .9 

Royal  Institution  Lecture 124 

Scope  of  Lectures 119 

Single  Phase  Motor 76 

Single      Circuit,     Self-Starting 

Synchronizing  Motors 50 

Spinning  Filament  Effects 168 

Streaming  Discharges  of  High 
Tension  Coil 155,  163 


Synchronizing  Motors 9 

Telegraphy  without  Wires.    . . .  346 
Transformer    with    Shield    be- 
tween Primary   and   Second- 
ary  113 

Thermo-Magnetic  Motors 424 

Thomson,   J.    J.,     on    Vacuum 

Tubes 397,402,  406 

j   Thomson,  Sir  W.,  Current  Ac- 


cumulator    471 

Transformers  : 

Alternating 7' 

Magnetic  Shield 113 

Polyphase, 109 

Rotating  Field 9 

Tubes  : 

Coated  with  Yttria,  etc 187 

Coated    with    Sulphide    of 

Zinc,  etc 290,  367 

Unipolar  Generators 465 

Unipolar  Generator,  Forbes,468,  474 

Yttria,  Coated  Tubes 187 

Zinc,  Tubes   Coated   with  Sul- 
phide of 367 


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